An investment round is a way for the company to aim for a higher future level than they would otherwise achieve. That's the moment when the shares are presumably the cheapest, and there is already a deal in progress so it is very easy and convenient to roll in another transaction. Since the lawyers are getting paid anyway and money is flowing at an established valuation it is an excellent moment to offer your shares.

Later on you might have to re-establish a valuation and you will have to make a lot of overhead on a relatively small transaction.