Thunderbird has few features that make it superior to proprietary mail client (MS Outlook)

True multiple pop3 client support

Using Thunderbird one can setup multiple email accounts as it can have multiple inboxes, one for each account. Outlook has only one Inbox for multiple email ids, meaning the emails will come into one inbox, leaving you to sort it all yourself.
In Thunderbird since each email has full client functionality, one can have a distinct signature for each id as well etc. So if a user has a Gmail 'personal' mail account and an 'official' id, she/he can operate both as full feature email clients using Thunderbird in a seamless manner.

Mail size and backup

Thunderbird uses the open mbox standard to store messages compared to the proprietary technology of Microsoft for Outlook (and Outlook Express) which stores all mails in one .pst file. Thunderbird stores each folder separately/distinctly on the hard drive in a transparent manner, making backups and restores of specific folders much easier. For backup just copy one folder and entire email and settings are instantly backed up and restored and available to use.

Cross platform friendly

Thundernird works well with all the platforms windows, mac, opensource. Bacause of this feature migration becomes hassle free, which is not the case with Outlook.

Mail Merge

Mail merge in Thunderbird is vastly superior. One can have email attachments in Mail merge, use BCC ids etc which is not possible in Outlook mail merge. It is also simpler, since you only a spreadsheet apart from Thunderbird, while in Outlook, need to use other applications as Word, Excel apart from mail client.

Search Feature

The advanced search feature of Thunderbird is very good with a host of options not found in outlook. It is similar to the advanced file find in the file explorer in GNU/Linux as well.

Security

The security features are better compared to Outlook, as users are protected from fishing as well as from email scams. Thunderbird contains built in spam filter, a "Bayesian" filter, which means that you "teach" the filter how you want it to behave by clicking on messages and selecting the "Junk" button at the top. The more messages you select as "Junk," the better the filter becomes at identifying spam, or at least messages that you consider spam. Outlook Express does not contain a built-in spam filter. Thunderbird's automatic updates keep the client updated to the new spam that is floating around on the web.

Speed

Thunderbird is faster copared to outlook. Outlook freezes the computer while downoding emails with bigger attachments often, not the case with Thunderbird.

RSS Feed

For those who use RSS, Thunderbird has an RSS reader unlike Outlook. This enables users to receive updates from blogs and whatever else they should subscribe to. This also makes life easier since all the updates they want will be delivered right to their in box.