[hhandoko]

It's fine when you are targeting one browser with a specific version (e.g. developing an internal web application with IE8 baseline). However, when you are building a web application targeting various browsers and versions, there will be subtle differences that may 'break' your UI.

CSS frameworks like Bootstrap provides you with a tested baseline, so you can be confident that the front-end you created will look the same across a wide range of browsers.

Certainly Bootstrap won't be suitable for all scenarios, but starting from a framework is generally a good idea.