The way it happened to me, it was just totally unexpected and I decided to go with it, no strings attached. Don't think too much about it, take it as an experience, you have nothing to lose after all (subjective, depends on how much you invest in prep time and if you lose work hours from your current job, so YMMV). I took it as a learning experience, I read books before interviews but did not go too 'serious' into anything, just the desire to learn and compare myself with my peers. I found the interviews to be really challenging (in the good way), questions were interesting and taught me a lot of things, both personally and in regards with peer interaction.

I would definitely recommend applying to these companies just so you can say "Hey, I tried and did it" (did it = applied, not necessarily got hired). The getting hired part is just a secondary thing, it's a personal accomplishment that may come or not, but it shouldn't necessarily be your end goal.

At least this is how I feel and how I felt after interviewing at Google.

Full disclosure: I am currently an intern at Google after having failed the full-time interview process and decided to get a Master's degree, which found me back at Google as an intern.