To effectively set the DC (once) for potential future opposing checks.
But, you don't set the DC once. If two people come down the road into my ambush, I make two hide checks, each opposed by individual spot checks.
Note, none of this precludes taking 10. I can certainly take 10 on both hide checks considering that I'm not particularly threatened and have had time to get nicely settled in.
I cannot take 20 on opposed rolls since there are consequences of failure (I'm seen). Similarly, I cannot take 20 early on and store it up.
But, taking 10 is no problem. I'm putting in an average attempt with the assumption that that is good enough. You can take 10 on most checks really. There's nothing really preventing me from taking 10 on a disable device check, other than the fact that it might be a really bad idea.
Scenario: I know that you will be walking down the road in about 10 minutes. I decide to jump out and surprise you. I'm wearing camouflaged clothing and I duck down behind some bushes just beside the road and wait for you. If I understand you correctly, your argument says that, at this point, despite being camouflaged and concealed, I'm not "hidden" because you haven't actually walked down the road yet; I can't use my Hide skill because there's no one around to Spot me. My argument says that, because I have concealed myself, I have by definition used my Hide skill and should therefore make a check (thereby setting the DC which you oppose with Spot whenever you approach).
Actually, you have not used your hide skill at all. Hide skills are an opposed check. Full stop. What was the DC for your hide check? To successfully use a skill, you must beat a target DC.
SRD said:
Some checks are made against a Difficulty Class (DC). The DC is a number (set using the skill rules as a guideline) that you must score as a result on your skill check in order to succeed
Now, it goes on further for opposed checks:
SRD said:
An opposed check is a check whose success or failure is determined by comparing the check result to another character’s check result.
So, what was the DC for your hide check? How can you say that you have successfully used a skill for which there is no DC?
If I can hide without a spotter, then I should be able to pick a lock without a lock. Or use diplomacy by myself.
Without a DC, whether a fixed one or an opposed one, you cannot use a skill. What you can do is describe your actions and possibly give yourself a circumstance bonus if and when the skill is used, but, you cannot use a skill until such time as you have a target DC.