It is a comprimise! If they need to exchange on X date, fine but completion is 2 weeks later, if they want to complete on Y date, fine but exchange is 2 weeks before.
And it is "kind" to lay your conditions out from the start so others know where you stand.
Solicitors will often sit on signed contracts for weeks. If they have until completion to exchange, they will put it off until then. If they have to have it dons before it can be!
It rarely suits anyone for everyone to be exchanging, completing and moving all on the same day. Even if there are 100% mortgages or BTL ISAs a gap is still possible, but solicitors will not offer to do that extra work unless you tell them to!