In BGP why do we use BGP route reflector?

To avoid full mesh connectivity between the routers working within AS under BGP protocol.

To avoid full mesh in IBGP

we use BGP route reflector when we have a BGP Route Reflector RR, instead of configuring full mesh BGP

Route reflectors (RR) are one method to get rid of the full-mesh of IBGP peers in your network

route reflectors are used to avoid the problem of full mesh iBGP peering, when the peers are quite large in number, as with the case of big enterprises or service provider networks. all the iBGP routers then would only form peering with the route reflectors, which would reflect the routes to other peers. say for example, if there are 10routers, it would form 10*(10-1)/2 = 45 peerings in the traditional meshed way. with one route reflector and remaning 9 normal iBGP routers, it would greatly reduce to just 9 peers. so the total peerings have reduced from 45 to 9, this reduction is more significant when there are hundreds of routers. but the only key point is that, we need to ensure the router refelector is capable of handling many peerings, as the network grows

BGP route reflector is a solution full mesh. the function of route reflector is to reflect all route to its ibgp neighbors but traffic will flow through tcp path.