Client versions are highly secured and useful for corporate as well as home users. It facilitates also to stay with emails in offline mode. Example: Webmail: Outlook.com Email Client: MS Outlook versions like 2013, 2010, 2007 and pre versions In MS email client, all messages are stored into PST file i.e. located at local system.
Another thing really important issue when using Client Based Email, due to the nature of the email being housed locally on your device, is if you computer crashes and you have not backed it up properly or recently, all those emails will be lost and as far as I understand it, unrecoverable. :/