The RD provides the capability to support address overlapping in different VRFs on the same node.
If both VRF A and VRF B have the prefix 10.10.10.0/24 how to discriminate between them ?.
In addition to this, the RD is a component of the VPNv4 NLRI
VPNv4 NLRI = RD:<IPv4+prefix> + VPN MPLS label
The route target provides a color to the VPNv4 route and allows remote PE nodes in importing to decide if the VPNv4 prefix is of interest for one or more locally defined VRFs or for none.
The route target is a BGP attribute of type extended community, the RD is part of the VPNv4 NLRI definition.
The use of differentiated per PE RD values for the same VPN allows route reflector to propagate multiple paths to client PE nodes that if using the same RD would lead to a single best path propagation (the best path would be likely the source PE with the lowest BGP RID if the IGP cost is the same)
Note: the above applies to a VRF site multihomed to two PE nodes for greater redundancy.
Multiple route targets can be associated to a VPNv4 NLRI but only a single RD is part of it.