Creating custom formations and playbooks was the single biggest factor that helped me really grasp the game. Once I understood how I want to build offenses and defenses I was able to improve my recruiting to fit my game plans and formations.
I have designed my depth charts so that every available player gets playing time and rest when their skills aren't being maximized. I do this by assigning them the top spot on a specific depth chart setting:
For example: On my depth charts here are my personal settings ---
QB = my passing QB that is typically at least a Jr* or Sr*
QB Rush = my QB on rushing downs that is typically underclassman that doesn't have the formation IQ yet to throw a pass
QB U1 = specialty QB for some scrambling plays in various playbooks
RB Pwr = my rushing RB that is my best RB and typically a Jr* or Sr*
RB Spd = my best receiving RB that isn't my RB Pwr typically a Jr* or Sr*
RB Poss= my second best receiving RB typically an underclassman
RB U1 = my best deep threat RB for specialty formations
FB = my underclassmen usually start here at Fr* and So* by blocking on rushing plays only
WR Dp = my best WR typically at Jr* or Sr*
WR Poss = my second best WR
WR U1 = my third best WR
WR U2 = my two youngest WRs are U2 and play only on rushing plays
TE Poss = my best receiving TE
TE Blk = my second best TE and usually an underclassman
OL U1 = Rushing Line
OL U2= Passing Line
OL Tackle/Guard = Short yardage rushing/best 5 offensive linemen
So by having this set system I can create new formations and edit playbooks to make sure I am maximizing my offensive talents.
All of my rushing plays use my rushing QB, my RB Pwr and FB, my TE Blk, WR2 x 2 and OL U1 x 5
My standard passing plays use QB, RB Spd and RB Poss, TE Poss, WR Dp and WR Poss and OL U2 x 5.
Using these settings I can control playing time and starts for underclassmen while still maximizing my team's talent
I have several specialty formations that I can easily build because of this depth chart system.
For instance I have a decent scrambling play that I will use QB U1, RB Pwr and FB, TE Blk, WR Dp and WR Poss. I will have the FB and TE pass block and send RB Pwr, WR Dp, and WR Poss on long/deep routes and if they aren't open my scrambling QB can get great rushing yards sometimes.
On the next play I might have similar personnel but actually it could be a unique formation utilizing specific players skill sets trying to achieve something else. I might run a pass play where I want my best RB to be the primary receiver and the TE to be the secondary receiver. I will fill out the skill set players in the formation to best help the team. I might give WR Dp a rest and assign WR Poss and WR U1 as the wide receivers if I find that my WR Dp (best WR) needs more rest than only resting on running plays.
I have a few formations that I have designed where I am trying to isolate a specific receiver vs a specific defender. I can easily add or delete these formations from my current playbook without having to mess with my depth charts. If I look at an opponent's FS or SS and think they can be taken advantage of - I already have a few specific formations where I am trying to isolate my best receiver at a certain depth vs the opponent's worst defender.
I do something very similar on defense with my depth charts but it is a little more complex - but same principle: everyone gets playing time while my defense's talents are maximized.