The @data, whats that?

Hey peeps I got a question in the code below:

mov ax, @data
mov ds, ax

or somethimes:

mov ax, @data
mov es, ax

I just know that these codes copy the OFFSET of data segment to DS and ES registers. BUt for what purpose? I hav these codes in my ASCII CHART program, and w/o it I will get ILLEGAL OPERATION on EXIT (running via dos box). Are these codes used for string operations, or just to access the offset of DS properly. Pls, explain this to me! Tnx! (or a little tutorial about this and other related stuff) Your help would be greatly appreciated!

PS: AsmGuru62, wer are u?? I'd been mailin' u, for about thrice, but no answer. Are u on vacation? Anyone can tell me where he is now? Tnx peeps

Cheers.