I never really used --alt-preset cbr until the other day when I encoded an audio track for a DVD rip.

I analyzed the file with Encspot, and couldn't help wondering why LAME had added both Xing and LAME VBR tags to the file .

It doesn't make much sense to me. I encoded again with -t and all was good.

Explain to me...

Not so much an explanation, but clearly someone made a conscious decision on this:

Extracted from lame.c code.

Can't really figure out the code, but I still wonder what's the purpose of this.

The 2 '//' effectively delete the 2 lines of code that would have prevented the writing of the tags. The tags were/are written by default unless switched off. So now, they're always written.

the LAME tag provides some useful information even in CBR case, like replay gain values. http://users.belgacom.net/gc247244/extra/tag.html

Well, you can consider this as an info tag and not a vbr tag. It is there because it includes a lot of information, like parameters used, encoder delay (allowing true gapless if there is support on the decoder side),...