Well, block gives you extra armour, Ready lets you try and hit them after they have acted. Defensive, offensive.
If you are up against an opponent with a high damage, hard hitting attack, you may be in a situation where you just want to live through the attack - when outnumbered by a couple of Draugr and Deathlord the extra passive Yellow - and potentially adding your shield value on to your armour (assuming you have something better than a wooden shield) - can mean the difference between staying up and quaffing a potion or going down under a hail of Honed Nordic weapons. Block, of course, also lets you Shield Bash which, if you are outnumbering an opponent, can be very handy.
On the other hand, if you see a pack of wolves up ahead, you know they are faster than you, but are fairly weak. Taking a Ready action lets them come to you first, and you can maybe strike and kill one before it tries to sink its teeth into you.