default build options

I am following the book "Absolute Freebsd" I suppose many of you will know it. I'm in the chapter that talks about the kernel, where you can remove drivers and options that you don't need. Well I have the following problem and I can not understand why, (I am a newbie) For example on my desktop...

I see DTrace Probes offered as a compile-time option/flag on many ports, and it's selected by default on the very vast majority of the ports for which it's available... As far as I can understand, though, DTrace Probes is just a debugging facility... I never programmed enough to really get...