I use a combination of SourceTree and the command line for my workflow, and can heartily recommend it. Some operations are just more intuitive to me when using the GUI such as:

  - staging portions of many files scattered across a directory tree
  - quickly glancing over diffs for a given commit
  - understanding the commit graph and the relationship between local and remote branches

Each developer's experience may vary, but in general I feel like a good GUI tool can help a novice user develop and reinforce the mental model necessary to understand what git is doing. For example, what am I doing when I rebase my commit onto master vs merging it?