The advantage is getting 2 or maybe 3 good skills at the same time if you just wait. For instance with a Rook, if you start with towers, archer tower, save, tower 2, and , you can then get boulder roll AND tower of light at one time at lvl 5. Better than waiting until lvl 6.

On some builds you can get 3 of your lvl 15 skills at once if you've saved you skill points.

If you go with a no-towers rook it's possible to have maxed out hammer (with dizzying slam), max boulder roll, poison arrows, and god strength 3 at lvl 15. From there you just boost stat points. Skills > stats pretty much every time.

There's no reason to invest a point in a useless skill(with respect to your current build or situation) when you can invest it in a useful skill 1-2 levels later.

wht everyone else said. 

  skills "unlock" at certain levels.     Say your 3 best skills all unlock at level 15.    Some people believe its better to save 2 skill points and so when you are level 15 you will be able to unlock your 3 best skills, and you wouldnt waste skill points..     Otherwise you would have to wait till level till level 18 to unlock your best skills and you would be wasting skill points.