Ostensibly, if "They" are "future humans" and provide help to the "present humans", the "present humans" survive into the future to become "They", who then provide help to the past. This is known as an ontological, temporal, or "bootstrap" paradox. On a linear timeline, it is impossible: if "present humans" can't survive without the "future humans", the "present humans" would die and thus there would be no "future humans" to save them. However, in a non-linear or cyclical timeline, this would theoretically be possible since there is no true beginning or end to the timeline. It is also possible that the "present humans" do survive to become the "future humans" without outside help, but the outcome is more detrimental to the human race, and the "future humans" are attempting to help the "present humans" to alter the timeline in favor of a better solution - one that they never initially had.
Another interpretation: Murphys Law: anything that can happen, will happen. This may be an indication towards the quantum mechanics theory of many worlds. This simply means that our daily actions, our free will, creates multiple realities, multiple branches of existence, many worlds, multiverses. You could for instance be served tea and coffee, and elect with your free will to choose coffee; however, in an alternate branch, you pick tea. This creates two realities or branches. Hence, every action, every invocation of our free will leads to a creation of new a branch of parallel existence, leading to possibly infinite branching like a fractal.
This choice making ability, this free will, which creates multiple branches of reality, can be illustrated as follows: http://rosettacode.org/mw/images/a/a3/Fractal_tree_bbc.gif In one branch, humanity suffers blight, the Earth is dying and extinction looms. In another branch, humanity has progressed, evolved and transcended above space and time. The transcended humanity, or referred to as "they", are able to traverse time as a physical dimension. Like normal humanity, "they" are able to travel through space backwards and forwards, and in all other directions. However, the difference is that "they" can also travel to events in time whether backwards or forwards. This time-travel is not a literal one whereby previous events can be altered creating paradoxes. Rather, all events which can happen, have happened. Murphys Law from this perspective has happened, not will happen. All possible potential branches of reality have occurred. All events across space and time as observable by "they" have already occurred, and are available for "they" to traverse; "they" are able to observe all branches. Since all possible events or outcomes, or branches of humanity are available for "they" to traverse, this implies that branching is in fact finite and not infinite. Therefore, normal humanity experiences limited free will as opposed to unlimited free will; i.e. choices are limited even though a certain route branches to transcending space and time. Since the branching of humanity and creation of many worlds is limited and finite, and since blight is threatening to destroy an entire branch of creation, "they", decide to rescue normal humanity to ensure the continued branching of many worlds.
Why would "they" want to create branching / many worlds? Well, "they" are humanity which stemmed from normal humanity in the first place from an earlier branch. Perhaps love is the one thing that transcends space and time and "they" desire to love? Perhaps "they" feel being alone and one is lonely perhaps the 'one' requires the 'many' to express love? Many and one have several themes: many worlds via quantum mechanics, many outcomes explained by Murphys Law, and quite literally many worlds visited; i.e. Miller/Mann/Edmunds. Nature is created and manifested in fractal branching patterns (http://www.miqel.com/fractals_math_patterns/visual-math-natural-fractals.html), by ill-executed actions of limited free will, normal humanity threatens destruction of a fractal branch which "they" decide to restore? More evolved, "they" have transcended time. If time is transcended, free will cannot exist. If time is a physical dimension which allows "they" to navigate to all events past, present and future; this presupposes all events have already occurred and "they" simply move from event-to-event, from one point in time to another. Everything that can happen, has happened. Therefore, "they", even though evolved, no longer have free will to create further events, thereby are no longer able to create further branches of realities.
Hence, the fractal branching stops for "they" due to no free will, and the fractal branching in another route is also stopping due to blight. Perhaps, this is another motivation for "they" to rescue normal humanity to ensure continued evolution? But, if "they" have no free will, how are "they" able to create a rescue event? The rescue event is created by "they" in normal humanity's branch of existence; i.e. placing of wormhole nearby Saturn, and a tesseract nearby/within Gargantua. Normal humanity then devises and pursues plans A and B. The tesseract placed in the bulk of normal humanity's space-time allows Cooper, via gravitational messaging, to interact with normal humanity's space-time. This interaction is expressed additionally with time as physical dimension on top of normal humanity's branch so that Cooper is able to traverse and message normal humanity's space-time events. Continued evolution may be a theme: "Mankind was born on Earth, it wasn't meant to die here" / "We're explorers, not caretakers" / etc. "Our destiny lies above us" may also elude to predestination, hence themes of free will.