Do I need an outlook .ost file?

All my emails are stored on our exchange server, yet on one of my machines, there is an .ost file that is 2 GB in size. There is no reason at all to have this, as far as I know. What is the right way to delete it? Why does it even exist?

The OST file is the offline folders file used when you enable Cached Exchange Mode.

Disabling Cached Exchange Mode should make this file go away if you don't want/need it.

The benefits of Cached Exchange Mode are in case the server is unavailable (either on a remote client, or in case of an outage) user can still work in their mail. Once server communication is reestablished, changes will be synced from the cached file.

The problems with Cached Exchange Mode are that in certain types of outages, the Cache may take on information that is incorrect once the connection is reestablished and will need to be disabled (to delete the cache) and then reenabled to begin caching again. Also, the file can be very large, usually at least as large as the mailfile, if not larger.

Aha, I got it. The file is just there for convenience so when I'm offline I still have access to my Outlook data. It's not critical for me to have, but handy, so I'm moving it to my D: drive so that is no longer clogs my C: drive which is for OS, not data. Here are the instructions I followed.

http://office.microsoft.com/en-us/outlook-help/move-an-offline-outlook-data-file-ost-HA010378297.aspx

To facilitate this, I made a folder on D that mimics C:

D:\Users\\AppData\Local\Microsoft\Outlook