External seed mucilage, which causes the seed to swell up in a sticky goop when wetted, is an extremely common trait across plants. It has evolved at least 100 times independently, interestingly, often in xeric habitats or in aquatic plants - that dichotomy certainly suggestive of multiple functions!
Seed mucilage is an economically-important trait, for instance, the mucilaginous seed husks of Plantago ovata make Metamucil/Psyllium supplements, chia seeds, a highly mucilaginous Salvia species, are used for their mucilage in baking, cooking, and breakfast foods, and flax seeds (Linum) are used in many breakfast cereals and granolas because the mucilage is a good binder.
Ecologically, not a whole lot is known about the roles of seed mucilage. Lab studies have shown that it can both inhibit and promote germination, itmay be useful in both epizoochorous and endozoochorous dispersal, it may lubricate the radicle during germination and establishment, and it surely changes the microbial community around the seed. Our work has focused on the defensive aspects of mucilage, once wetted and dried, it either binds sand and substrate tightly to the seed or strongly cements the seed to the ground. Both of these aspects reduce seed predation!
A thousand interesting ecological and evolutionary projects could be done to look at the functional importance of seed mucilage in nature. If this is of interest to you, let me know: the lab would love to have smaller undergraduate projects, larger grad student projects, and even postdoctoral projects focusing on seed mucilage.