It all goes back to evolution. Some fish do not require scales like sharks. Most of it boils down to where the fish lives salt or fresh, rocks or sand and how many other fish there are that will have him for lunch. That isn’t always true however.

The skin of many fish is covered with scales that protect a fish like roof shingles protect a house. You could also compare the covering of scales to a suit of armor.

A fish only develops scales during its first year of life. As fish grow, their scales get bigger. Fish growth, and scale growth, slows in the winter and speeds up in the summer. The alternating periods of growth form rings on a scale just as they do in tree trunks. It’s possible to examine these rings under a magnifying lens to see how old a fish is.

All fish have body slime. The species with less well developed scales or no scales at all tend to have more slime. The slime reduces drag to help a fish move faster in the water. It also provides protection against parasites, covers wounds and helps a fish maintain the proper balance of salt in its body.

Since sharks do not have many natural preaditors, they don’t need a suit of armor. (most of the time they lull around at appx. 1.5 mph looking for injured fish and are opportunistic feeders) Cat fish due to their spine also do not need to have a suit of armor. Some smooth skinned puffers have an alternate means of defense, they blow up.

Sharks certainely do not use their tails covered in skin to taste so why would his body be covered entirely in these dermal denticles? They rely on their sense of smell like catfish which are the best at smelling in the water. A cat fish can detect food in as little as 1 ppm. A keen sense of smell helps sharks find food. Through touch, hearing, and sight, they detect movement in the water. A special organ on the snouts of sharks, called the ampullae of Lorenzini, allows them to detect weak electrical voltages from living creatures or ocean currents. they do not tast with this gland or their skin.

Most fish are covered with scales to help protect the body. Some species like catfish have bony plates which serve the same purpose, while others are protected by a thick layer of mucous.

Many species have very small scales.

Also, many species of fishes lack scales. All the clingfishes (family Gobiesocidae) are scaleless.

Sharks on the other hand have a covering of dermal denticles to protect skin from damage and parasites. This also helps to improve fluid dynamics.

~Hope this helps!

Most fish have scales and it is a form of protection that evolution gave them against things like other fish, larger particals in the water and anything they brush against. It is also handy in IDing eachother. Fish will look for similar fish and scale patterns are one way to know who to breed with (like birds with feather color).
Most of the fish with skin are bottom dwellers that need a slimmy protective coat to fight off bateria and fungus that they would otherwise be suseptable to (why catfish can live in really nasty places) Also skin gives them more flexability for manuvering around and through rocks. Scales wouldnt have quite enough flex.
Sharks have skin as an evolutionary design both for protection and detection. It is ruff like sandpaper and they will use it to investigate things before biting. They will rub against an object and see if they can detect blood from the micro cuts inflicted by their skin. Also it allows for the flexability they need to make sharp turns and to glide against the reefs without having to risk scraping a scale off and becoming suseptible to disease. Great question!

thats like asking why some peoples skin is black and some white? thats the way god made them

I’m pretty sure it has to do with regulating heat in cold blooded species.

Not God’s plan,EVOLUTION.

I think more primitive fish don’t have scales. Scales are part of evolution