Ill keep it simple,
If you cant land within 60 percent of the runway available at the destination then you need an alternate. You MUST be able to land within 70 percent of the runway at the alternate. Once you have the alternate you can plan for using up to 80 percent of the runway at the destination.
The alternate just provides you a 20 percent relief from the 60 percent rule. This is just one of the reasons one would require an alternate in 121 operations.
1 No instrument app. at destination
2. Supplemental Ops. (charter)
3. 123 rule
4. 60/70 rule. see above.