In Romania it varies, but you get a better deal (small discount or get a parking spot, better orientation etc.) when you pay down 20-40% of the price before construction starts so many buyers go for it.
Waiting until the construction is finished usually means many units are sold (the good ones).
It also depends on the builder’s reputation of course, if it’s a big name they can charge more in advance.
IMO it sucks a bit for the buyer as delays are quite often and hard to attack in court.
It also attracts speculators who sign the pre-contract just to sell for a profit when the units are delivered.
Literally how every condo project in Asia is sold. Not sure how it is like in Europe.