A question regarding synthesis problems: How do we decide when to use H3O+ as an acid catalyst as opposed to H2SO4? I see some reagents like HBr use H2SO4 while others like LiAlH4 use H3O+.

H2SO4 is can do duel work
1) It acts as a proton donor to give H3O+
2) It acts as dehydrating agent which helps in removing water from the reaction mixture.
Many reactions are reversible and when water is one of the product ,then H₂SO₄ is the best choice.
It helps in removing water as the product and  shifts the equilibrium more in the direction of the product(Le Chatelier Principle).