Question

what is the role of fuse used in series with any electrical appliance why should a fuse with defined rating not be replaced by one with a larger rating

Answer 1

Fuse is used for protecting appliances due to short-circuiting or overloading. The fuse is rated for a certain maximum current and blows off when a current more than the rated value flows through it.
If a fuse is replaced by one with larger ratings, the appliances may get damaged while the protecting fuse does not burn off. This practice of using fuse of improper rating should always be avoided.

Answer 2

A fuse with defined rating for an electric circuit should not be replaced by one with larger heating since then the very purpose of a fuse wont be fulfilled that is to restrict the flow of excess current that can cause problems

hope this helps :)