When a company goes public, they usually hire an investment banking firm to manage the process. The I-bank does a lot of work to properly value the firm and price the IPO right, but part of their job is also drumming up interest among potential buyers of the stock. Like any securities offering, the IPO is basically an auction where the amount of money you raise, and the terms you raise it at, depends critically on how many people you can get interested in your offering and how excited they are.

One way to incentivize the I-bank to do their very best to get the highest price for the shares, and to keep the price high, is to make them shareholders themselves. Thus, Microsoft did a small (Crunchbase reports it as $1M) financing round with their investment bank right before IPOing, despite not needing the money at all. Any dilution is more than made up for by the better IPO price; basically, the I-bank got to share in their upside by making their upside bigger.

funding where higher risk takers get paid more but "controlling rights" remains with smaller invester(s), so I guess MS didn't want to give up any control by selling shares, other then select few.