This is maybe a obvious question but why a client need to use a rendez-vous point instead of directly use a introduction point for communicate with the hidden service?
It is because introduction points are published in HsDir and consequently a attacker can more easily deanonymise clients and servers if the introduction points are used for a long-term communication?
There has been a proposal to do this for some time, the proposal also includes the rational of why they are two seperate relays currently. Ultimately it has not been implemented and given that the newly created "next generation" onion services, Rendezvous Protocol v3, still uses distinct Introduction and Rendezvous Points, it's unlikely to be seen in the near future and given that the proposal hasn't been touched in almost a decade, it may never be.
From Proposal 142: Combine Introduction and Rendezvous Points:
There are some reasons for separating the two roles of introduction and
rendezvous point: (1) Responsibility: A relay shall not be made
responsible that it relays data for a certain hidden service; in the
original design as described in [1] an introduction point relays no
application data, and a rendezvous points neither knows the hidden
service nor can it decrypt the data. (2) Scalability: The hidden service
shall not have to maintain a number of open circuits proportional to the
expected number of client requests. (3) Attack resistance: The effect of
an attack on the only visible parts of a hidden service, its introduction
points, shall be as small as possible.
