i am learning building Dapps. And i got to know to know about metamask. I've read about metamask is an ethereum wallet which we can use to sign Tx's without running a full node and inject web3 to webpages. and I know that the architecture of Dapps is like-
web-app --> web3 ---(JSON RPC)--->RPC PORT ---> GETH NODE--->ethereum API ---->ethereum blockchain.
if I am running my own ethereum node. and in my web-app i am using nodejs which can import web3 library.so why do i need metamask? and where does the metamask fits in Dapp architecture?