A lot of different answers to that, many commercial software houses like Serif and Borland use an obsolete version as bait, download the free version then hopefully you'll give in to their blandishments and pay for the current edition. Anti Virus companies like alwil and Grisoft are a variation on a theme, it's partly a marketing exercise, they don't have the distribution network or advertising budget of McAfee or Symantec so are using word of mouth from satisfied free home users to introduce their products to those commercial users who will pay for the full licence. They also will expect a percentage of free users to upgrade to the full license, just to get those extra embellishments that could make life complete ( I don't see much point, the advanced features lost in the free home versions are not necessary for home use. )
Others are simply posted by programmers who don't expect payment, they just want to share their efforts. Some of these programs stink others are as good as the commercial equivalent, best way of getting them is through sites like Snapfiles click here , where they have evaluated and rated the available downloads.