I thought separating two vowels was why we use fusion consonants, but why do we need to use one before the suffix -le ? Because there isnt two vowels that need separating :S

because suffix -le is a postposition actually (ile=with) which is becoming a suffix but has not completed its mutation yet.

So after a word that ends with a wovel it becomes -yle/-yla.

The same fact is observed for idi/imiş/ise (-ydi/-ymiş/-yse).

Over time we may expect that that fusional consonant will disappear. In fact in some areas (İzmir for example) -y- is dropped for -ydi/-ymiş/-yse but not for -yle yet.

iyi-di instead of iyi-ydi

iyi-miş instead of iyi-ymiş

iyi-se instead of iyi-yse

Edit:

-yle/-yla has many variants in daily language similar to -ecek/-acak suffix

nine ile dede = grandma and grandpa

nineyle dede

first -yle is added an -n

nineylen dede

then l mutates to n

nineynen dede

and also fusional consonant is dropped if there is an n at the end

ninelen dede

similarly

araba ile gitti = he went away by car

arabayla gitti

arabaylan gitti

arabaynan gitti

arabalan gitti

All these variants can be heard in daily speech.

Thanks for the reply...its clearer now