Thinner = lighter weight. Batteries, hard drives/SSDs and other components are smaller. Laptops are not the best medium for high performance computing (i.e. gaming, complex programs, etc), they are meant for portable computing. When performing the tasks that they were designed to perform, they do well. It is when you stretch them they have issues.
The simple answer is you buy a product and use it for what it is designed for. Lightweight and thin are great if you are traveling around, need access to email and light web surfing - maybe editing a word document or spreadsheet. Not as great for gaming, photoshop, rugged environments, etc....