Part 121 carriers need destination alternates using the 1,2,3 rule.
1 – 1 hour before and after scheduled arrival time
2 – If the ceiling (broken or overcast layter) is less than 2000’ or,
3 – Visibility is less than 3SM.
Some carriers also require an alternate if there are excessive crosswinds.
So if you have a good weather day, you do not require an alternate and you get to save carrying some fuel.
Edit: Damn, you bet me to it. At least I explained the 1, 2, 3 rule for this guy.