You can’t delete a user, but you can deactivate an account so a user can no longer log in to Salesforce.

If an user is inactivated/deactivate in Salesforce, then

1. He will not be able to login.

2. Report will not run, if he/she is set as running user.

3. Web to lead and web to case will not work, if he is set as the default creator.

4. User license of the inactivates user will be free for other user.

5. Deactivated users lose access to any records that were manually shared with them, or records that were shared with them as team members. Users higher in the role hierarchy relative to the deactivated users also lose access to those records.

6. Deactivated users are removed from the default opportunity and account teams of other users. The deactivated users’ default opportunity and account teams are not removed.

7. If a user on an account team has Read/Write access (Account Access, Contact Access, Opportunity Access, and Case Access) and is deactivated, the access will default to Read Only if the user is reactivated.

8. If you deactivate users in an Organization where opportunity splitting is enabled, they aren’t removed from any opportunity teams where they’re assigned a split percentage.

9. If the running user of a scheduled job is inactive then it will fail with either of the following errors:

'First error: Inactive User, orgId: 00DA0000000XXxx userId: 005A0000001xxxx'
'First error: [INACTIVE_OWNER_OR_USER] owner or user is inactive'

This is mainly done, when the user leaves the Company/Organization.