Question

for example cyndaquil learns quick attack at level 13 and as an egg move why

Answer 1

If you want to use pokemon in the little cup (which has level restrictions) you'll still be able to use certain moves. Plus, you might want to have a move on a pokemon earlier on. Using Cyndaquil as an example as well, If the right parent knows the TM move flamethrower, then Cyndaquil will also get it. It's much nicer to get it early at level 1, then say, level 37 (assuming you don't even evolve him)

Comments

Also,because somtimes moves used for In Game purposes can be different from Competitive purposes,so,like DT pointed out,if it's your first HG/SS and you chose Cyndaquil,and maybe by mistake,or for some other purpose you removed Flamethrower,there's allways the chance to breed to  regain the move.