Legs adds a lot of additional mechanics for explorers. Foot exploration, caverns / caves, mineral deposits fauna / flora reveal.
Legs also adds additional options to facilitate ship repairs from the inside. It also could allow proper co-operative crews aboard ships, if players can opt in to manage parts of a ship. Different ships would have varying accommodation for crew quarters.
Elite is a superb space game, adding space legs just enhances the freedom of choices one is presented with in order to indulge in this ridiculous crazy galaxy backdrop we all greatly admire.
Whilst legs need not be a mandatory game component, it would for certain add a level of immersion few games out there provide.