Lenses, as opposed to many camera bodies, are long-term investments in your photography kit. Since 2004, I have owned six camera bodies as my primary workhorses (probably not many by some professionals’ standards, but enough), and a few more as backups. In that time, I have owned only one version of every lens I have purchased. I bought my first really fast glass in 2005, a used AF-S Zoom-Nikkor 17–35mm f/2.8D IF-ED, and I don’t see myself selling it or trading it in anytime soon. It’s still sharp, functions like new, and I think it flares less than newer ultra-wide lenses. It is easily the oldest lens I own, and the rest of my “trinity” isn’t too much younger.
The point is that lenses can offer you many more years of photography than a digital body. Depending on the model, a newer version of a camera body can be released what seems like every year. Lenses, because the technology does not change as frequently, have much longer shelf lives than cameras. Even though camera bodies cost more than lenses in some cases, the glass is where you see your kit really develop due to its longevity.
Lens investments are informed by our own personal, professional, and stylistic needs. Want to upgrade that kit lens immediately after purchasing your first DSLR combo kit? Spend a bit more time with it to see if the investment is necessary. I’m not one to encourage going into debt over photography; instead, consider your purchase options and your own style/needs to make an informed decision about the lenses you acquire.