It depends. Picard is really built around the idea that it can download you the metadata for your files from MusicBrainz. For this it provides you the tools to search and match your files to entries on MusicBrainz, and to automate how you want to get your tags written and files renamed using scripts and plugins.
But of course you can also edit the tags manually if that’s all you want to do. You can drag and drop your files into Picard, click on a file and edit the tags in the metadata box manually. Then save. Just make sure you have “Save tags to files” enabled in the Options menu. And unless you want it and have configured it to your liking leave “move files” and “rename files” disabled.
However, if your are only doing manual tag editing you miss out of much what Picard has to offer and also I think for pure manual editing other tagging software might be even better suited.