Ant said:
Hmmm. I can't see the sense in that. You'd still get paralax error and so you might as well use a separate viewfinder like more conventional point and shoots. :?
That's exactly what is going on; the TLR has a separate viewfinder, it's just not tiny. TLRs have a ground glass, which gives you an almost 100% full sized (size of the neg that is) image for focusing and composing.
Asking why would they make a TLR vs an SLR is sort of like comparing a prop driven plane and a jet. TLRs came first, and then when the technology was better and affordable, they made SLRs. Modern medium format SLRs are huge, early ones were gigantic.
EDIT: I guess it would be more like comparing a bi-plane with a single wing plane
Why use one today? Well, besides the fun of using vintage/antique stuff(people still love to drive their old cars, etc...) the main reason I'll pull out my Rolleiflex TLR instead of my Pentax 67II SLR (both medium format) is that the TLR is 1/2 the size and weight. A TLR doesn't take up much more room than a 35mm SLR (less if you are comparing a 35mm or DSLR with a big zoom). TLRs are very easy to hand hold. Although I've used medium format SLRs hand held, they are best used on a tripod.
I also like the waist level finder and ground glass for focusing and composing. It's like a DSLR or digicam with the big screen on the back. Much nicer than a small viewfinder. I don't know about all TLRs, but my Rollei is very well corrected for paralax.
Another big reason is price. You can get good TLRs as low as $75, with a nice Rollei with Planar or Xenotar lens for $300 to $500. Most used medium format SLRs with a lens and some accessories will run $800+.
If I was only allowed one camera to use for the rest of my life it would be a Rolleiflex TLR w/ Planar or Xenotar lens. It's my favorite all around camera. It's portable, easy to use, and takes wonderful photos. I've used mine for landscapes, portraits, weddings, documentary, and even shooting bands in bars. And back to my first point, they are just cool and fun to use 8)