Some people buy Stellar because they believe it's a good financial investment, while others buy it to support a more equitable financial ecosystem. Many buy it for both these reasons.
As there is a finite supply of just over 50 billion XLM tokens, if overall demand for the token rises, then the price could follow. The demand can increase through speculators or people using the Stellar network and XLM token.
To buy XLM is to make an investment into a more humanitarian and equitable cryptocurrency.