Plate Tectonics is the theory that explains how and why Earth's lithosphere is shaped as we observe (semi-rigid plates of crust and mantle, floating on hotter, less-rigid mantle, splitting apart in some places, and colliding in others.
An analogy for the Earth’s crust is the skin that forms on the surface of a pot of slowly boiling custard/pudding. The rocky crust tears apart in some places.
It permits new pudding to succeed on the surface to form new skin (= igneous lava called basalt). Elsewhere within the pot, skin piles up (converges upon itself), and the molten pudding beneath it descends backtrack towards the bottom of the pot to heat up again, leaving the skin behind.
For tectonics, this convergence of crust-upon crust builds mountains. If enough crust builds up, deeper parts of that pile become insulated by overlying crust, and obtain squeezed by its weight, may become unstable and rearrange at the atomic scale to make new minerals through metamorphosis.
The hydrologic cycle, with sea water yielding water vapor through evaporation, then rain through precipitation, then runoff because the rainwater flows back to the sea, causes most of the weathering of rocks, and transport of sediments to low-lying areas (the lowest being the sea).