Marketing! In order for a cryptocurrency to be useful and worth something it has to be used by people and this is hard to achieve. With giving away free tokens you can get a network effect going. It's an easy way to build a large community because everyone loves free tokens. Another purpose of this is to draw people's attention to the project. Whenever they scroll through CoinMarketCap or see the logo of the free coin elsewhere they know it. This will eventually even buy them tokens, since the project appeals to them.

It's a win-win situation: On one hand, you get free tokens. And on the other hand, blockchain companies raise awareness for their projects.