Simply because Web performance really matters !
99% times it will give you faster end-user response times.
Here are a few exampels from Velocity Conf.
- Bing – A page that was 2 seconds slower resulted in a 4.3% drop in revenue/user.
- Google – A 400 millisecond delay caused a 0.59% drop in searches/user.
- Yahoo! – A 400 milliseconds slowdown resulted in a 5-9% drop in full-page traffic.
- Shopzilla – Speeding up their site by 5 seconds increased the conversion rate 7-12%, doubled the number of sessions from search engine marketing, and cut the number of required servers in half.
- Mozilla – Shaving 2.2 seconds off their landing pages increased download conversions by 15.4%, which they estimate will result in 60 million more Firefox downloads per year.
- Netflix – Adopting a single optimization, gzip compression, resulted in a 13-25% speedup and cut their outbound network traffic by 50%.
From Steve Souders, pioneer in Web Performance Optimization,
80-90% of the end-user response time is spent on the frontend - Start
But inlining is -sometimes- preferable for single page application or web sites with one single page view per session. There is no golden rule, and generally forget it as it concerns mainly very specific web sites really involved by end-user performance.
You can read here why performance matters (Disclaimer : I am the author)