It says a number of times in the Parsha (25:17, 25:36, 25:43), regarding various instructions of behavior, "And you should fear your God" - ויראת מאלקיך.
Every time it says that, it is referring to a situation in which it is difficult, or even impossible, to know whether the persons actions were done with good intentions or with self-serving motives. In these situations, the Torah reminds us that Hashem knows all and knows what is in our hearts, so even if other people don't know, Hashem still does, so we should do it for the right reasons.
It is easy to be frumm when everyone is looking at you and watching what you are doing. As a matter of fact, it is a common phenomenon that some people who are very frum, when they go away on vacation are much more lax and do things they would never even consider doing back in their own community. This is because they are comfortable with the idea that nobody is watching them and seeing them in this "less frum environment".
That is really when you need the fear of God - the Yiras Hashem. When you are away from your normal setting, around people who don't know you or care what you do. If you have yiras hashem, you will stick to your ideals even then.
Yiras hashem is what keeps you on the line, even when nobody else knows what you are doing.