Why does cron use MTA to send emails, while we use MUA to send emails?

I have looked at the definitions of MTA, MUA, MDA, for example, https://ccm.net/contents/116-how-email-works-mta-mda-mua and https://en.wikipedia.org/wiki/Email_agent_(infrastructure) - In Lubuntu, the default email client is Sylpheed, and many also use Thunderbird. If I am correct, both Sylpheed and Thunderbird are MUA. - In https://unix.stackexchange.com/a/479613/674 , Stephen mentioned that cron uses MTA (such as postfix or sendmail) to send the outputs of its jobs as emails. My questions are: - Why does cron use MTA instead of MUA to send emails? If cron can use MUA to send emails, how? - why do we use MUA (Sylpheed or Thunderbird) instead of MTA to send emails? If we can use MTA to send emails, how? - When we install MUA (Sylpheed or Thunderbird) , does Sylpheed or Thunderbird need a MTA installed on the **same** machine to send emails? Thanks.