Immutable

[Brad via Digitalmars-d-learn]

I was looking through lots of sample code on Rosetta Code.  D has 
a lot of solutions out there.  That is really nice but it has me 
wondering - coming from other languages that do not support the 
concept of immutability - do real world programmers and/or 
hobbyists really use it as much as I see it on Rosetta Code?  I 
know it adds a layer of security to your code, but I am still 
thinking "why?".

Thanks for entertaining a newbie question.