My question is about the purpose of metamask if it does not support token purchases and I still need to use a token management service?
Metamask is a Chrome plugin that allows anyone to interact with the Ethereum blockchain with having to run a full node. Just install it and you can start using supported dapps.
That being said. Metamask is used to buy tokens. You can use Metamask to interact with ICO smart contracts and it is used to trade Erc20 tokens, for example over EtherDelta.