Migration

In the case of POP accounts, the PST file format is used, while in the case of IMAP accounts, the OST file format is. It's not uncommon for email providers to use both POP (Post Office Protocol) and IMAP (Internet Message Access Protocol) to send and receive messages.

PST can be used on different device using Outlook and can be Import to read.

OST can not be used on different device and one can not import and read while its configured with server and connects with esp, in the case of esp it is offline data and for this we can make it export into different format such as PST that is POP3 and can be read by importing on Outlook and on other ESP supporting services.

I recommended you go through the below links to get the better idea about IMAP and PST. Hopefully, your doubts get clear.

https://www.magustools.com/blog/email/what-is-a-pst-file/
https://www.magustools.com/blog/general/what-is-imap/