What is the effect of switching noise?
What will be the effect of switching noise if I use that kind of supply in repair work ?

That depends on what you will use the supply for when doing repair work.

Can the supply be used as a replacement for an AC adapter?

Probably yes, as an AC adapter is also a switched mode supply and will also have some ripple on its output voltage. Most laptops run from 19 V from an AC adapter, this voltage isn't used directly. There are many switching regulators inside the laptop as well which convert the 19 V to 5 V, 3.3V and 1.8 V for example.

Can the supply be used instead of a battery?

Usually yes unless the circuit relies on the low noise / ripple free output voltage of a battery. It depends on the device if this is the case. Cases where the supply ripple might be an issue are audio amplifiers, radio receivers and sensitive measurement equipment.

Even then, when using a supply which outputs a ripple on its output voltage, the device might still just work only with audible noise or measurement equipment might not perform as accurately as it should.