I used to build rods, and when learning the basics, I learned a few things about guides. There are a few main elements to it:
1. Guides control the line during the cast. This is ESPECIALLY true with spinning rods, where the line whirls off the reel in a spiral. The first guide captures the spiral, then the next couple of guides narrow it down, until it is almost straight when it goes through the running guides and tip. On a conventional rod, you have some side to side motion, but not so much, hence the bottom guides are not nearly as big.
2. Guides transfer sensation to the blank. To SOME degree, guides transfer sensation to the blank, so you can detect taps, bumps, hits, etc, and you can feel when your lure is bumping bottom and so on. More guides transfer better, BUT if the guides are heavy, it can dampen the whole rod, so it's a balancing act. That is why the finest guides are as light as possible for their intended use. So alloy guide frames with SIC guides were the cream of the crop for awhile, but now there are even lighter guides.
3. Guides transfer the power of the fish to the blank. This is one of the other factors in how many guides you use. Theoretically, you could get away with only a tip guide. It would work, but then ALL the pressure would only be on the tip of the rod. If you only have two guides and a tip, it would have 3 points of contact and so on. The taper of the rod has something to do with how many guides you want. As mentioned above, a thick, broomstick-type rod doesn't need too many guides, because there is very little taper, and it's a very strong blank. A light steelhead/salmon rod with light action, but made for very heavy fish, tend to have many guides, to transfer the load across the entire blank.
4. Guides hold the line away from the blank so it doesn't rub. This is for traditional CASTING rods. When a fish bends the rod, if there aren't enough guides, the line will rub the blank in between the guides. You can make higher profile guides, but then the leverage of the taller guides will make the rod want to flip over. Because of this, many custom rodbuilders have gone to "Acid wrap" or "Spiral wrap" guide systems. This is where the lowest guide is on top of the rod, like a normal casting rod, but over the next 2-4 guides, transitions to the guides being on the bottom of the rod, like a spinning rod.
In my opinion, Acid wrapped rods are superior. Because of this transition, you can use much lighter guide frames, as the line only pulls on the guide, and is not trying to crush it like a typical casting guide. Hence, you can use a lighter weight guides, even single-foot "fly" guides.
My very favorite conventional rod has a 7'0" Mag Bass taper, XH power blank, with a Shimano Triton 100 reel, filled with 30# braid. It is spiral wrapped with 4 lightweight double-footed Alconite guides for the transition, and 5 fly guides to a light framed tip. This rod is so light in the hand, some guys almost worry, but it's a BEAST when you hook up to a fish. It's a joy to fish with, and for awhile, guys who tried it demanded that I build them one.
I don't build for anyone else anymore, but I still appreciate a well-built rod.
So in summary, the guides on a rod are important to SOME degree, but a quality blank and intelligent build are probably more important. Good guides can make the most of any blank, but they cannot fix a slow, dead-feeling blank.
So for a heavy catfishing or shark fishing blank, go with heavy and powerful, because you are not constantly casting and waiting for that subtle bite. For your finest finesse rod, where you are making long casts and drifting light lures in deep, fast currents, you want the best blank you can afford with numerous guides of the lightest build for the power you need.
