TL; DR; You need to clean!
You do this for safety, repeatability, and to avoid wasting your effort. I have cleaned poorly before and wasted brews of both wine and beer, since I took a more rigorous approach I have only had 1 contaminated brew in 13 years, and that was using kit and sanitiser that were not my own.
Even if you are making a wild brew you need to clean down and scrub out all the crap and then sanitize you equipment. You only want to get into your brew what you intend to get in.
Even in breweries in Belgium where they use coolships, they make sure they are clean before introducing the wort, and blowing the air over. They also ensure that the fans are covered with fine mesh to stop flies or leaves etc being blown into the wort. They only want the micro flora and fauna to inoculate their wort, not some residual contamination hiding in a valve or seal or sitting on a fly.
Yes, you need to do the tedious cleaning our you will get biological contamination, which can lead to a number of problems:
- Most serious you get some from of mold, which can have mycotoxins, that can kill people.
- You get acetobacter contamination which will turn your whole brew to vinegar
- You get wild sacchromyces/other yeast strains which can lad to phenolic or other off flavours
- You get lactobacillus which will produce lactic acid in weakly hopped beers, and will alter the sugar consumption profile of you primary sacchromyces strain.
- You can get pediococuus which will turn your beer 'ropey' and will acidify your brew.
- Enterobacter can contaminate your brews and turn them into vomit smelling horrific messes.
- There are others thing that can get into your beer and produce off flavours but those are the main ones.
Most food poisoning bacteria salmonella, e.coli o157 etc... can not survive in fermenting beer past day 3 the alcohol and pH drop does their job and kill them off. Mycotoxins from a mouldy seal seating or a poorly cleaned valves are the main health danger.
All the other will just give you unintended funk, higher levels of acid, or disgusting ruined beer.
Once you have had one of these contaminations, if you are using plastic fermenters or soft rubber seals, you probably should bin the kit, or at least let it soak in bleach solution for a few hours to thoroughly sanitize. You can take all your seals and sterilise them in a pressure cooker, for 30 min.
Soft tubing will need to be sanitized as well, Brett in particular is highly tenacious and can contaminate brews weeks later. Or cause over attenuation after bottling causing the dreaded bottle bomb scenario.
If you are in a professional brewery this is taken to a even more extreme level, all vessels, hoses and unions will be cleaned with a combination of caustic and PAA. Then should be routinely checked with a ATP Bioluminescence Meter if a result after cleaning is >30 you have a big problem, it should be <10 RLU.