Regardless what it's added for, fact is, it still thins 1 card from your deck. It still improves your chances of drawing what you need to be higher in the rounds after you use the decree.
That's a moot point because you (usually) want to use Royal Decree to get an expensive card from your deck, not cheap fodder in order to draw a better card later. Royal Decree is consistency foremost, but one distinction does need to be made (read on)...
Portal is not played to thin, Portal is played because of its power (hence people keep it for round 3).
This whole discussion is getting out of hand because people are comparing the same kind of cards in different scenarios. Let me explain the situation.
There are basically four usages for tutors, which can be used in conjunction with each other (not being mutually exclusive):
1) Tempo
2) Consistency
3) Thinning
4) Filtering
1) Tempo
Only applies to tutors with a body (including Portal and CotF). Put an extra unit on the board with extra points. Sometimes these points matter, but usually tutors are not used for tempo, with the exception of Portal
2) Consistency
Royal Decree being the prime example, when you need a specific card, you double the chances of drawing it. While Royal Decree also thins, it's usually not the primary reason because you play an expensive card rather than fodder. Incidentally, Royal Decree itself is not played for tempo, but the unit it summons can be. I wanted to make this distinction clear.
3) Thinning
Thinning is only needed if you want to draw into your combo in a later round and that, in turn, is only necessary if the said combo requires it, which only applies to (NG) Hyper-thin, to enable the Tibor + Vilgefortz package.
4) Filtering
This is the one most players forget about. Filtering is dropping cheap cards in early rounds in order to have a better chance to get expansive cards later. It's different from normal thinning because you aren't doing it for the sake of thinning, like with NG Hyper-thin.
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When you ask what role does Portal play? I'll say that it depends on the deck it's being used in. For NG Hyper-thin, it's about all four of the above. However, with Calanthe it's mostly used for tempo. Filtering is only a secondary reason. And, when you look at SY (mostly Dijkstra), Portal is used for consistency on top of the tempo.