The resource_link_id is a required parameter (both in 1.1 and in the new 1.3 specification). This is an opaque unique identifier that the Tool Consumer (TC) guarantees will be unique within the TC for every placement of the link. If the tool / activity is placed multiple times in the same context, each of those placements will be distinct. This value will also change if the item is exported from one system or context and imported into another system or context.
If the context parameter alone is always sent, then there is no way for a Tool to know what exactly is being asked for in the launch. Often a launch user doesn’t wish to always land at the header or top-page of a book or a course. Rather, it is preferable that multiple context links are available so that clicking (for example) Chapter 1 will result in a LTI launch directly into Chapter 1, bypassing the front page or the Introduction. That is the source and importance of the resource links – those are specific _places_ in a context for launch. In point of fact, the design is very portable as well, since it may very well be that a context is a collection of resources hosted by a Tool. The resource_link_id allows the Tool to know which thing is actually being asked for within the context, whether it is a separate chapter in a book or a separate resource across a digital context (say a map, or a detailed annotated photo).
The meaning behind making resource_link_id required is such that there is always confidence for the Tool that no launch should ever arrive incomplete.