It protects a buyer who cannot qualify for or afford two mortgages at once, letting them commit to a new house without being stuck carrying both if their current one takes longer to sell than expected.
The trade-off is real in a competitive market: a seller weighing two offers will often prefer the one without a sale condition attached, since it closes with more certainty, so a buyer using one may need to offer a stronger price to be taken seriously.
A bump clause, letting the seller keep marketing while your condition runs, is the usual compromise, and drafting the exact deadline and notice terms is part of what the standard $800 to $1,500 in legal fees on a purchase contract already covers. Get your hand-reviewed valuation from Joel Dyck.