The added maxima at the end of spells denotes a more powerful version of the spell cast by the wizard/witch. Lumos is not the only spell it is used for; another example I can think of is the spell Protego maxima used in Harry Potter and the Deathly Hallows. Protego is a shield spell used to protect the caster, whereas its more powerful counterpart is used to protect an area (such as the area that the tent is set up when used by Hermione, and Hogwarts by the teachers before the final battle). I don't remember if this spell is specifically cast in the books or if it was generalized as a "protective enchantment".
As Protego maxima is only ever used by the more advanced wizards in the movies, it can be assumed that it is a more difficult spell to cast than its regular counterpart. This is backed up by the fact that Harry is practising the Lumos maxima spell in the opening scene of Harry Potter and the Prisoner of Azkaban, and clearly finding it difficult to pull off.
The differing spells clearly also have different applications. One would not use the maxima version of the Protego spell to protect oneself in the moment, it would be impractical. In the same way, there is a distinction between the two versions of the Lumos spell: they have different applications. The minor version produces more directed light like a flashlight, whereas the greater spell will light an entire room.
For the Lumos spell that you specify, the distinction between the two is to create differently lit environments for the characters. As stated above, Lumos maxima would light an entire room. This would be used in scenarios to show that the characters can see everything lit in a room, as well as allowing the audience to get a full picture of the room.
The lesser version of the spell is used in the same way as a flashlight is used in filming, which has several applications. These are:
- when a character or the audience has their attention directed towards an object in the scene
- to ensure a character or the audience do not immediately notice salient objects within a scene
- to create suspense by not allowing the character/audience to see an entire area so that they can be surprised/shocked by something entering the frame.
All three of these are used in the scene in Prisoner of Azkaban when Harry is following the Marauder's Map towards where it says Peter Pettigrew is within the castle. His wand points towards the map in order to allow the audience to focus on the movements of Peter, and show him coming towards the character.
It is then used as a suspense technique as there is only a limited amount of vision of the hallway, so that it is difficult to see if anyone is approaching. Then when the unseen character is shown to leave in another direction on the map, the limited light ensures that the character and the audience do not have the reveal of who the character is, and why he has shown up on the map, as
Peter Pettigrew is also Ron's rat Scabbers.
This means that within the scope of the movie, both versions of the spell were needed. The original version of Lumos used within the book, in order to have this "flashlight" effect lighting technique for this scene (as well as others), and the "room-lit" effect, in order for the audience to see the full scene unfolding instead of a confused mess of limited light.