Understanding physics #1

I have a few questions. I like to ask why to get a better understanding because my textbook is just throwing out definitions and formulas.
Why do we need to identify and discover the nature of particles?
Why do we need to find out the energy changes in collisions?
For inelastic collisions, do we need to calculate momentum in each dimension using vectors and then combine the vectors to get the total momentum?
What is momentum actually used for?

momentum is the basis for newton 2, so
https://isaacphysics.org/concepts/cp_newtonii?stage=all
and its clearly related to the basic suvat
v - u = at
as multiplying through by mass gives
change in momenum = impulse
where the impluse is average force*time. When the force is not constant, the integral of force with respct to time gives the change in momentum over that time period. So (in gcse physics) when a car crashes and the velocity goes from 30m/s to zero in 0.3s, you can work out the average force without knowing in the specific details of what happens in that time period.
More description in
https://www.physicsclassroom.com/class/momentum/u4l1a.cfm

in an inelastic collision, restitution (change in ke) applies along the line of collision (normal to the plane of contact) due to the body(s) deforming. So you have to resolve into the two directions where restitution occurs / does not occur, then combine afterwards.
https://eng.libretexts.org/bookshelves/mechanical_engineering/mechanics_map_(moore_et_al.)/10%3a_impulse_and_momentum_in_particles/10.04%3a_two-dimensional_particle_collisions