Building science tells us there are 4 main elements to control 

(listed in order of priority)

1. Water (nobody likes a leak)

2. Air (keep conditioned air in, unconditioned air out)

3. Vapor (keep it from condensing in walls)

4. Thermal (keeping the temperature comfortable.

The link below explains this in greater detail.

Save on heating and cooling costs

Improve indoor air quality

Consistent comfort

Fewer Drafts

Prevent moisture & mold

Up to 40% reduction in outside noise

Defend against insects & pests