Why was Rachel holding onto Lavan's idols for a few days? Since she was trying to prevent Lavan from doing Avodah Zara she should have thrown them out immediately!
In this related answer, I showed that the answer to your question is rather controversial in the commentaries, and no answer is totally clear. One of the thrusts in the commentaries (Rashi, Rav Hirsch etc.) is that she stole them to wean her father off idolatry, by demonstrating that they were powerless.
The most direct answer to your question would be along these lines, and in the words of Rabbeinu Channanel (brought by Rabbeinu Bachaye):
ותגנב רחל את התרפים אשר לאביה; she stole them in order that he would reconsider his actions, saying to himself that any deity which allows itself to be stolen surely cannot be much good to anyone. The same argument had been used by Yoash who said if baal was really capable of avenging himself, he should do so himself instead of letting his human worshippers become his defender (Judges 6,31). Similar arguments are reflected in Ezekiel 28,9 where the prophet predicts the downfall of the King of Tzor (Tyre) who had declared himself a god. He ridicules this “god” as saying to his murderer: “I am a god!” (quoted by Rabbeinu Bachya).
It seems that he is saying that if they were destroyed, then fine - there are ways of accomplishing that would have made sense to Lavan. But them being stolen is something that would make him realise that they are powerless.