There are different methods that emails can arrive into our system if we use a mail client, such as outlook or thunderbird, to read our email messages.
Even though, you are using Microsoft Outlook (program) to access these emails message, there are generally 3 methods which email is accessed and downloaded from the email server.
IMAP Protocol
For modern email clients, if you only keyed in your email address and password, generally the protocol will be set as IMAP. It is a 2 way communication or synchronization of email messages among the email servers and your device.
Mails downloaded will remain in the email server’s inbox and all your devices inbox. If you have created folder to organize your emails, the folders will be reflected among the rest of your devices.
Eg, you have moved your email from your boss into the “Boss” folder. The same email will be in the “Boss” folder when you access other devices to read the email. Mails sent from your device will be in the sent folder of the email server too, since they sync with each other.
Or if you have deleted the junk mail from your inbox, it will be cleared from the inbox in other devices as well.
This protocol is suitable for you if you have multiple devices and you preferred your folders’ organization to be mirrored.
The core advantage of using IMAP email data is stored in the server.
For instance if your existing PC or phone crashes, you do not need to worry about your email data. All you need to do is to purchase a new device, and reconfigure the email under IMAP protocol and they will re sync back to your phone
Disadvantage of this protocol
However, any important items deleted from your email folders will be deleted among all your devices which is using IMAP as well.
This protocol requires a large webmail capacity if you wish to keep all your emails synchronization among all your devices. As any deletion of email to free up space, will be deleted among all your devices too.
Generally if the webmail storage capacity is full, you will not be able to receive any new emails.
Exchange Protocol
It works similarly to IMAP, except that other than syncing email messages and folder structures, it syncs your contact, calendar too. However, this only works in Microsoft Outlook email client and for emails server hosted by Microsoft. Eg Office 365, hotmail.com outlook.com etc
POP Protocol
As explained above, IMAP mirrors the email messages and folders into your email client. POP is however, a one way communication from the email server. Once the email message landed in your inbox, it will have no relationship with the email server.
For instance, if any emails deleted or moved in either the server or the device, it will not be reflected. Eg if you delete the email in your device inbox, or created new folders to organise the data, these changes will not be reflected in the web server. If you received spam messages and you have deleted from your device, it will continue to stay in the webmail.
** Additional Info **
If POP Protocol is selected, you will have an option, if you wish to delete the email upon downloading into your email client, or to remove after a number of days, up to 99. Read here for more info. 
Advantage of this POP Protocol is that if your web server has limited space, you can safely clear the emails in the server by logging into the web mail portal and your downloaded mails will remain in your PC, if they have been downloaded, prior to deletion in the web portal.
The core disadvantage is that data will not be synced. So if you are the type who like to organize almost all your emails and prefer the folder organization to look alike, IMAP could be a better fit.
Another disadvantage is that you will need to backup your own pst file, as data might not residue in the web server.
Conclusion
Considering that modern email servers, Office 365, Gsuite, hotmail, yahoo, generally offers a bigger email capacity, up to 50GB. And most of us has multiple devices (work laptop, mobile phone, home pc), IMAP will be a more suitable choice for the modern worker.