It's needed because of governments, and lets be honest here people do not want a coin/asset which its price flucuate alot.
Ok lets make an example here, lets say I get about $1000 a week for my job. Now lets say the world changed to using BTC right?
1) will you give me a SET number of btc each week? i.e. 0.008 BTC a week? If this is the case then thats dangerous because what if BTC dumps to 100 dollars tomorrow?
OR
2) will you give me a number of btc that is equivalent to my $1000 which I had received weekly regardless of the price of btc? In this case there's other problems that can arise, you can speculate on a few.
Now for purchasing from the vendors is another issue. How would we price stuff? is it based on our dollars equivalent, or will we have a pricing TAG purely on BTC? I don't know, lots of things to work out.