The PST is now a very old file format. Originally introduced to give users the ability to store email moved from their Exchange mailbox, the need for PSTs disappeared years ago because Office 365 users have massive mailboxes. Despite this, some organizations persist in allowing people to use PSTs. That's OK if you don't want to take advantage of Office 365 compliance features or have access to the data via OWA or mobile clients, or lose it when the files corrupt. Overall, it's just better to move valuable data into online mailboxes and eradicate the PSTs.