For fear of a down round - but most don't think that far. Basically let's assume a company raised money on a $10B valuation, and needs money down the line for

For fear of a down round - but most don't think that far. Basically let's assume a company raised money on a $10B valuation, and needs money down the line for a variety of reasons. They will have to make sure that the future round is at $10B+ which may or may happen, especially if the company is not flying high at that time. And, if they can't raise at $10B+, they'll have to go for a down round which can look very bad.