Why do browsers still spoof user agents? -

i know browsers spoofed user agents in order allow feature detection. wondering why still so. don't think user agent spoofing has place in modern era of standards compliance; browser nirvana web developers compared situation during infancy of web.

someone it's backwards compatibility old code out there. reason? after time think browser vendors looking beyond sites old code. being worked on, or these user agents forgotten relics tougher times browsers?

additionally, feature detection these days seems done javascript, makes part of feature detection use case user agent irrelevant.

because nowadays have multiples possibles user agents iphone5s, galaxy siii, ipad 2 , on. because necessary handler site features in different ways specific rules example.

think in scenario user requirements that:

• the site should able chat customer tablet.
• in mobile should not able because smaller.

thus, because have multiples devices, have handler in different ways give user great experience.