There are a bunch of actual legitimate use cases for stupid powerful lasers, aka class 4s.
Laser medicine has a whole bunch of uses:
http://en.wikipedia.org/wiki/Laser_medicineDon't try these at home.
There are many different applications in manufacturing:
http://en.wikipedia.org/wiki/Laser_beam_machininghttp://en.wikipedia.org/wiki/Laser_cuttingThere's potential home use here; a relatively benign application is a 2D router laser cutter. Personally I'd start out with something a bit easier like putting a dremel in there, lasers add a legit scary fire hazard to the equation. Containment is also a high priority, because you need to make sure that a stray reflection is in no way putting your eyes (or other people's) at risk.
This brings us to our first PPE side note; ANSI Z136, learn it, love it.
http://en.wikipedia.org/wiki/Laser_safety#RegulationsQuick primer on PPE:
http://en.wikipedia.org/wiki/Laser_safety#Safety_measuresYou want wavelenght and power specific eye protection in all cases with class 3B or above laser, and skin protection for class 4.
You want quality, certified PPE. For example, lower class 4 lasers would make me consider at least OD6 protection to cover accidental exposure:
http://www.lasersafety.com/eyewear/polycarbonate/p5e01Prolonged exposure, higher power and other variables will affect the OD rating required. You are also required to put in similarly rated material in any aperture or viewing window on your laser cutting table.
DO NOT TRUST YOUR SAFETY OR THE SAFETY OF THOSE AROUND YOU BY USING CHEAP, UNCERTIFIED PPE OR OTHER SAFETY EQUIPMENT. If you are unsure of what you need, join a relevant forum and ask for resources and advice.
Beyond that there are plenty of other uses for lasers, but the need for a class 4 laser is debatable at best. A class 3 or even 2B will be plenty for the amateur science dabbler. Same goes for any sane astronomy uses. Definitively stick to class 2B for presentations or messing around with your pets.
Beyond that, just going outside at night with a modest 5mW green laser is tons of fun, so long as you stay courteous to anyone else around you, aka not fucking pointing the beams at people to scare them as certain people *cough*the idiot seller*cough* suggest.
When you've gotten a feel for the various safety issues and what specific project you want to take on, then you build up. Just going 5mW->10mW gives you an appreciable gain in performance, but as you approach 100mW you need to start to seriously think about containment because you have a beam that is an imminent health hazard at short range and a possible distraction tens of miles out.
Going out and just getting a big fuck off class 4 laser without a) a specific application, b) a functional understanding of safety issues and PPE, and c) actual use of proper PPE is beyond insane.
Add to that a whole bunch of issues you have no idea even exists, such as how much IR you laser is spilling. Cheaper models will even have unaligned/ineffective/straight up absent IR filter, which means you could potentially be putting out an order of magnitude more power than you expected, and if your PPE doesn't account for this secondary output wavelength you are in imminent danger.
I could go on and on but tl;dr if you don't know what you're doing, grab a 5mW 532nm green laser to play around with, join a laser enthusiast community and build up from there.