Paid plans either enable or give you longer version history 6 months minimum, some businesses bump that up to 1-2 years (it goes to 10 years with dropbox).

Same with google. Paid plans you can make sure a copy of all sent messages is kept, set retention rules etc. So if folks use email for all their approval / business flows that can be helpful (ie, some orgs CEO might send an email say I approve - go ahead and sign it on big deals rather than shuffling paper or doing a proper e-signature. If so, you might want to keep an archive org level of your email flows