You can make use of a lookup later in your feature file, in order to avoid repeating the feature code.
(For instance, you might want the same substitution to be active in multiple different language systems. First, you wrap the feature code in the lookup; later, you just call the lookup again.).

http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html#4.e

Lookups are also useful for isolating more complex contextual substitutions. I've always used them for beginning and ending substitutions: Scroll down to Karsten's post:
http://typophile.com/node/19839

Ok thanks. It is a little bit clearer. But I need to include lookups in case I have some issues with complex situations. If I have some basics alternate characters to replace and if my salt feature do the job fine, do I really need to add it ? I mean I can live without it (if my fonts features works fine), right ?

Yes, the font will also work just fine without lookups.

The feature file compiler will take care of building lookups behind the scene, I think allowing defining features without lookups is merely syntactic sugar.

Ok thanks for all the precious infos ! And the prompt answers.

A thing I forgot to mention:
Lookups will go and jump back to the beginning of a line before doing their job.
Consequently, by including a handful of lookups, you can actually make more complex substitutions happen than would be possible with a single-glyph treatment.

That is what I understood. But as my font is only replace this glyph by this glyph without any contextual swapping, I'll dig into lookups when I really need it :D

I've tried to explain what lookups are and how they are used here:
http://typophile.com/node/29898#comment-171706

Regards,
Adam

great thanks adam.