Deploying smart contracts and interacting with DApps on Tezos requires XTZ. The cryptocurrency pays for computations and data storage on the network. In this respect, XTZ can be deemed a utility token. Similarly, its unlimited supply distances XTZ from those cryptocurrencies with finite issuance. Fixed supply coins, like BTC, have more in common with other investment vehicles.
That said, XTZ may still represent a good investment. It's impossible to use the network without its native asset. Increased usage will create demand, which should put upward pressure on the price of Tezos.