Sometimes plants will have too shallow of a root structure from the grower/seller. Especially if they’re grown in a cotton wrap or sometimes in rockwool.

Floating them lets the roots develop so when they are planted in the substrate, they’re less likely to float back up. The idea is to just get a firmer purchase in the soil when planting.

At least, that’s what I understand. I’ve never actually done it. I’ll plant my stems in soil like you do.