Why are you using micro-service architecture?

I am a noob programmer who has just entered a cloud based project and I am wondering why is everyone talking about cloud functions (GCP) and lambdas (AWS). I mean what happened to the old ways of developing software when it was hosted on a single server and it was accessible by various clients from anywhere.

Why is the trend of micro-service architecture in such demand that people are leaning more towards these big cloud related services?

Is it really that much cost effective in the implementation standpoint? Or is it just a hype, a buzz word just to attract clients to spend more money?

I really want to get a in-depth knowledge on this trend.