Forwarding adjacency is the feature that has the IGP advertise TE tunnels as links.
P routers are able to run IGP adjacencies over these TE tunnels, and thus able to direct packets to these TE tunnels.
Forwarding adjacency allows all nodes in IGP domains too see the TE tunnels and use them in the SPF calculation.
“Autoroute announce” is another method to forward packets into the TE Tunnels. However “Autoroute Announce” only affect the local LSR. It does not announce the TE Tunnels into IGP as links.
You do not want to use “forwarding adjacency” or “autoroute announce” all the time. For example when you want to control exactly what kind of traffic will be routed over the TE tunnel, you do not want to let’s IGP to decide which packets will be routed to the TE tunnel, and thus you do not want to configure “forwarding adjacency”.