Re: why do we measure active contents of chemical i.e. surfactant ?

It's pretty straightforward. The active content is generally the part of the surfactant solution that actually is the surfactant; for example, a solution in water of sodium lauryl sulfate that is described as "30% active" means that 30% by weight of the solution is SLS, and 70% is (usually) water.

This can be important if you're treating a textile in a bath, for example, and want to know what level of the chemical in the wet bath gives what level of pick-up in the finished, treated textile. It can also be important if you are treating the textile to neutralize or remove something (e.g., formaldehyde or a dyestuff), where you need to calculate how much of the actual chemical you need to react with the thing you want to eliminate.