Reason for Thunderbird usage  

When I last used it- a good many years ago- everything was local. I have read the Help documentation
MailStore Home Help

for you and can find no specific statement about that, but if you read this:

Backup and Restore - MailStore Home Help

it's apparent all the files are on your PC.

Remember there may be other data to transfer e.g. contacts, mail rules...

Not everyone uses the major free email providers like Gmail, Outlook, Yahoo, AOL and iCloud that have a slick user experience.

There appears to be the following use cases for Thunderbird users

• People who run websites (webmasters)
• People who want ownership of their data
• Employees who were assigned an email address by their employer
• Students who were assigned an email address by their university
• People whose ISP provided them an email address but their website has a poor user experience
• People who need an email address for business purposes (not personal)

If you run your own website you can host your emails yourself, if you want to have the ownership of your data, or you might be using an email address assigned to you by a company.

If you run a company or website and are using email for business purposes, you'll receive lots of emails to reply to from customers or users and the free major email providers like Gmail, Outlook, AOL, Yahoo, iCloud etc will not be suitable. Because if you send too many emails in a specific time period (eg. one hour, day, week or month), you'll exceed their fair use policy so you'll be blocked from sending any more emails until your time out ends. People use free email providers to send spam, fraud and phishing emails and their zero cost attracts much more criminal activity than if someone paid to host (or run) an email address themselves. If an email provider or website has too much "outgoing spam" being sent by their users, they will end up on a "email blacklist" so the receiving of their emails will be blocked, and in serious cases their website will be suspended so it will be gone from the internet.

Also if your ISP gives you a free email address and their online website has a poor user experience, Thunderbird will provide a much better user experience.

In this case, Thunderbird is a good software to access your emails using the IMAP protocol and also access your contacts and calendar with CalDAV and CardDAV protocols.

I know that Google and Microsoft are targeting business customers through Google Workspace and Office 365, but that's not the point. Even so, it hard to decipher what unique selling point they provide over me paying for a web hosting account that has the cpanel control panel out of the hundreds already out there, and using an email address from that.