Directed LDP is not a term that appears in the LDP specification (RFC 5036.)
Perhaps you're trying to understand the difference between Basic Discovery which uses LDP Link Hellos and Extended Discovery which uses Targeted Hellos. Targeted LDP is for exchanging messages with a distant, non-adjacent peer.
Targeted LDP can allow routers to use LDP-signaled ...
In todays routers routing lookup is done in hardware, usually in TCAM, which is as fast as ILM lookup.
So no advantage in lookup speed.
So why is it even needed? For tunneling.
If you want to deploy a full-mesh of tunnels dynamically between routers (usually PE routers) you can use LDP. This provides infrastructure for services like L3VPN, L2VPN etc.
Since the LDP is running using TCP, it needs to establish session with every LDP speaking neighbor. Sometimes those neighbors are one hop away and you can use the point-to-point addresses configured on interfaces, but sometimes those might be a few hops away (e.g. when using L2 vpns/EoMPLS) and then both routers must be able to discover each other first ...