Alright,
Mine you this is a brief Tile and Grout intro so bare with me and you can find alot more info by doing a search and checking out the Gold Mine:
Tile Cleaning 101.
Typically the oils and greases that make up the majority of soil on tile and grout need to be treated with an alkaline style cleaner to break up, and emulsify the soils.(same as carpet pre treatments) So most alkaline tile cleaners are high pH because of heavy degreasers. Also most good tile and grout cleaners should be able to rinse easily with water as most pressure pump chemical inject systems do not work well at high pressure levels.. ( unless specfically noted).
The acid side comes into play when you have grout staining. Alot of grout on the market is a sand based mixtures utilizing colorants dispersed through the mixture. The color goes through the entire grout line. When you have surface soiling/staining, if you can etch the top layer off, you can usually reveal a clean non stained grout line. The big issue here is only etching a small amount. And most acids made for tile and grout cleaning are designed to do just that after an alkaline cleaner has removed a majority of the soiling.
So alot of jobs with sanded grout you its not uncommon to rinse it twice with a spinner tool.
Some people will also use oxygen releasing agents like oxiblaster in place of an alkaline/acid combo cleaning and alot of grout will respond to these bleaching agents as well as a means of improving the appearance of the grout.
The problem with acid is it attacks the calcium in the grout. The wrong acid will penetrate and keep attacking all of the calicum components and thus you can actually weaken the grout as a result.
Certain stones like Marble, travertine, limestone are calcium based as well and will etch and erode when acids are used as well. This is most notieable on a polish stone floor. And most stone when exposed to an acid will react to some degree depending on the mineral content of the stone. So this is why you have to be careful with acids. They will also etch stainless steel as well causing big problems.
Now as for the comment about the grout not needing to be sealed. There are new grouts out there are are expoxy based and some that are a combo of epoxy and other minerals that contain sealers in them. These grouts are pretreated and really should not be sealed as a topical sealer can haze or react with the sealers naturally present. Also you need to be careful and preinspect for this type of grout because many of the companies producing these grouts will say big no no to spinner style cleaning.. One example that we will sometimes use in outdoor tile and grout:
http://www.starquartz.com/
Also preinspect for previous color sealers that have been applied poorly, broken grout, loose tiles, and anything that may become a problem during your cleaning.
P.S. ask Scott W. for his free guide to cleaning stone tile and grout!