Why use String.Format?

Why would anyone use String.Format in C# and VB .NET as opposed to the concatenation operators (& in VB, and + in C#)?

What is the main difference? Why are everyone so interested in using String.Format? I am very curious.

Solution

I can see a number of reasons:

Readability

string s = string.Format("Hey, {0} it is the {1}st day of {2}.  I feel {3}!", _name, _day, _month, _feeling);

vs:

string s = "Hey," + _name + " it is the " + _day + "st day of " + _month + ".  I feel " + feeling + "!";

Format Specifiers (and this includes the fact you can write custom formatters)

string s = string.Format("Invoice number: {0:0000}", _invoiceNum);

vs:

string s = "Invoice Number = " + ("0000" + _invoiceNum).Substr(..... /*can't even be bothered to type it*/)

String Template Persistence

What if I want to store string templates in the database? With string formatting:

_id         _translation
  1         Welcome {0} to {1}.  Today is {2}.
  2         You have {0} products in your basket.
  3         Thank-you for your order.  Your {0} will arrive in {1} working days.

vs:

_id         _translation
  1         Welcome
  2         to
  3         .  Today is
  4         . 
  5         You have
  6         products in your basket.
  7         Someone
  8         just shoot
  9         the developer.