Yes so is Outlook Express built into windows but I prefer Outlook as I did try out T-Bird.
From my perspective Thunderbird has 2 advantages over Outlook:
1/ Cross platform. If I switch Operating Systems I can take everything with me.
2/ Non proprietary data storage. This allows easier transfer of stored messages to another program if you should switch.
I like non proprietary data formats so that I can choose what software works for me without worrying that my stored data will be lost or "distorted".
I would like to see Thunderbird have an Inport/Export function to make moving my stored messages easier.
My current stored messages go back several years and have been transferred to a new computer at least twice under Thunderbird. I won't say it was easy but I was able to do it.