You may want to buy 1 lottery ticket just to be in the lottery. You are right. That costs you only 1 satoshi and you won't lose anything. But generally speaking, it's not a good idea to buy lottery tickets.
In the last round of weekly lottery, there were around 189 million tickets and the prize pool was around 0.256 BTC. This means that  buying lottery tickets is actually like playing a game with the house edge of around 86%.