I've got an 'el cheapo Wahl beard trimmer that I bought from walmart about 5 years ago for $15. Still works like a charm. I have pretty much a full beard going if I don't shave every 3 days so I think I can lend some help here.
With me, if I just use the trimmer on my face, it will leave you stubbly. Meaning, the hair isn't long enough that you could grab it with your fingernails, but you still can see the darkness of it on your face and kind of gives you that unkempt look that you get if you don't shave for a day or two.
Putting the guard on, and taking all the way down to the lowest position will *almost* trim the hair down to the skin, but it will be just long enough to maybe be able to grab it with your fingernails.
One click longer on the guard and you will actually have some length to the hair and it keeps things neatly trimmed. I always use this setting to trim down and tidy up.
Never really keep my facial hair long enough to take the guard any higher.