Why we need to dilute bleach before use as a disinfectant?

Short answer, bleach is poisonous.
It has to be diluted to make it safe to handle.

Long version:
Bleach is a solution of a solution of sodium hypochlorite. The chlorine fumes are very strong. When inhaled, it can cause a build-up of fluid in the lungs and severe shortness of breath that could lead to death if untreated. Immediately or within a few hours after breathing chlorine gas, the lungs can become irritated, causing coughing and/or shortness of breath.
Bleach is very irritating and corrosive to the skin, lungs, and eyes. It has been known to burn human tissue internally or externally. On top of this- it may cause skin rash, extreme headaches, migraines, muscle weakness, abdominal discomfort, esophageal perforation, nausea and vomiting. In addition, it can damage the nervous system.

Therefore, it's best diluted to the point where it is still effective against bacteria, fungi and viruses.

Surfaces should be cleaned of general dirt with detergent and water first.

The second cleaning with bleach destroys the proteins in living organisms.

The recommended concentration of bleach for routine disinfection of cleaned surfaces is 1000 ppm as this concentration has been shown to be effective against the majority of microbial pathogens, including blood borne viruses.
(Ref: CDC Guidelines for Disinfection and Sterilization in Health Facilities, 2008).

I use it full strength in water. The fumes are toxic. I have to be careful because I have reactive lungs and can smell things quite sensitively.

Too effective. Pretty dangerous. At much less concentration... it’s still very effective.

Full strength is less effective than a 10 to 1 solution. It has been a while since I was trained on that but it had something to do with the ease of the diluted solution penetrating the viral cells.

The ideal concentration is 0.12%. Straight from the bottle is 2-10%. The recommended dilution has the same disinfecting power but is less risk to your eyes, skin and lungs.

Because at full strength it discolors things.

Bit caustic for disinfecting your hands

It’s to stretch what you already have