Thunderbird has a few benefits which other Mail-Clients are missing.
First of all and as base-feature without installing any add-on I'd like to mention the treeview that is able to sort mails chronical by item:
Two more features need installing an extension for each.
The first feature is standard in Outlook and perhaps other Mailclients: Writing a Mailresponse the old Message is headed with Details about the old Message. I wanted a correspondending Message like in Outlook and not the Standardmessage of Thunderbird even if it's possible to change it in the about:config-file.
Ok, I installed the Add-On Change_quote_and_reply_format and now I can configure my own message with
Extras->Add-Ons->Change_quote_and_reply_format like you see in on the next image on the left side.
The second feature is really cool: Loading the Add-on EditHTML it's possible to edit the source-code of HTML-Mails you want to send. So you can insert styles, JavaScript and special handmade Code. You can see it below on the right side.
Without these three benefits I didn't like to use Thunderbird as Mailclient because Outlook did the job well, but editing source-code of HTML and the treeview are respactable arguments for Thunderbird.
There are a lot of more Add-Ons for Thunderbird, so you can rebuild a Mail-, Communication- and Datingplattform like Outlook with some more special features.