Siacoins are used in lots of ways but serve a single purpose – facilitating network operations. When you upload files to Sia, you pay for that storage in Siacoins. If you're a host and you're storing other people's data, you get paid for that in Siacoins. If you own Siafunds, your share of Sia network growth is paid out in, you guessed it, Siacoins.