I don’t know why Thunderbird can’t get a reliable, functional search ability. It’s such garbage. I constantly have to delete my entire search index and start from scratch, it is immensely frustrating.
Maybe see if Betterbird’s search works better for you
It lets you have all of your emails offline as well. If you have to reference an older email, it’s faster than loading the webpage again.
Some desktop email clients lets you manage your emails, contacts, tasks, and calendars all in one program, which loads immediately instead of loading multiple web pages. This is why I love Evolution and Thunderbird.
If you have multiple email accounts, it’s easier to use an email client, rather than having to log into multiple websites.
The search function in some web interfaces suck.
Some people just don’t like their email provider’s web interface.