it is interesting to read opinions and experience of each others about such and such gear piece...Needed or not...
But there can be only anecdotes here , because too many factors and specific needs and different gear levels experience are implied...
I dont need a preamplifier with my integrated TOP Sansui alpha for sure driving my headphones only now ...
But suppose i want to upgrade it with a TOP power amplifier because i had mew big speakers , i will need one....Or suppose i decide to buy a TOP integrated superior to the Sansui alpha ? Now suppose i upgraded this integrated superior to my Sansui alpha with a power amplifier top in the world , i will need a TOP preamplifier ( a tube Berning preamp ? ) ...
This had no ends, add each one of us system and needs and all acoustic, electrical and mechanical parameters at play...
There is no rule here AT ALL ....
By the way as i said i bought a low cost tube preamplifier to add to my active speakers : total improvement and not small ...It is anecdotal as is anecdotal the possible improvement in some specific mechanical,electrical and acoustical embeddings workings system/house/room if we throw off the preamplifier ...some coupling are bad or useless or simply not synergetical etc ...