It should in principle be possible to infer the continent of a subject from the country it is in—unless the country itself is part of two continents, as is the case for Russia, Kazakhstan, Caucasus countries, Turkey, Egypt(?), and certain nations with overseas territories, in whose cases specifying the continent on subnational entities may be appropriate. As India is part of only one continent, in my view there is thus no need to add the continent property to anything which is a part of India, since the continent which it is on can be inferred by following located in the administrative territorial entity (P131) some number of times (or following country (P17) once) to the item for India (Q668), which is appropriately noted as being in Asia.
There are some queries I can come up with that take advantage of the relationships formed with P131 and P17, but even though almost all of them time out with the Wikidata Query Service (i.e. they require more than one minute to run), I am sure that if you got around to adding "continent: Asia" to every place in Asia, a query that relies only on the continent property would definitely time out.
There are lots of uncodified norms and customs on Wikidata—which, I assure you, is as confusing to you as it is to me—so I have started a discussion on Project chat to help clarify this.