There have been many keyword abilities through the years of magic the gathering but they generally have a few categories that they fall into. Combat abilities, casting cost effects and flavor. Phasing while an interesting ability falls into the third category and a little bit of the second. This isn't a bad thing as some of the most interesting abilities are largely flavor based but it does mean that if you are not getting any other value the phasing ability is nothing but a weakness.
Ertai's familiar is a rare illusion from the weatherlight expansion set of magic the gathering. A time when rares did not have to be good. It costs one blue and one colorless to put into play and is a 2/2 creature. This is not unreasonable for a blue creature, except that this creature has phasing. This means that during your upkeep if he is in play he goes into exile and if he's in exile he comes into play. In addition when this happens to this card you put three cards into your graveyard. To counter this you have the ability to pay one blue mana, if you do then Ertai's Familiar won't phase out until the end of the next turn.
All of this leads to a creature that you only get half of the time unless you are willing to pay mana and one that is no larger than it would be in general without the disadvantage. So why would you play it?
The only possible answer is that you want cards in your graveyard. There are a few good reasons for this but some of the easiest to consider are flashback, reanimation and threshhold. The problem with all of these is that Ertai's familiar is not all that fast. Once you cast it you won't put any cards into your graveyard until the beginning of your next turn. And then it will be two full turns before it happens again.
There is even less advantage to this card in limited than in constructed because in limited there is no good reason you would want cards in your graveyard and with a 40 card deck it is much more likely to deck you as well.
Modern magic players are spoiled by having rares that are generally playable in most packs they buy. This was not always the case. There used to be a great many nearly unplayable rares and Ertai's familiar is one of them.