Having a 16vs 18 in a particular stat is the difference of +1 in modifier. It's only a 5% increase, but if it is your primary stat it does get used a lot more than a secondary stat. I've heard (but have no personal experience) that the Living campaigns tend to harsh and expect strong primary stats.
Actually its 5 percent points, not 5%.
If you usually hit 50% of the time, a +1 gives you 10% better chance to hit (it will now be 55%). Or, to say it another way: if you do 20 attacks and usually hit on 10 of them, a +1 will make you hit 10+10% = 11 times.
Since 4e is balanced around you hitting about 50% of the time, thinking of +1 to hit as a +10% damage bonus is quite accurate.
That +1 will also increase your damage by one (you do your primary stat to damage 90% of the time). For a cleric it will typically be 1d8+3 or 1d8+4 with 18 instead of 16.
1d8+3 * 50% = 3,75 dmg
1d8+4 * 55% = 4,675 dmg, or 24,67% more damage per hit.
Going from 16 to 18 in your main stat does a LOT!
