It comes down to tempo: getting a 4/5 body for 1-2 mana in the midgame is a great tempo play, because it's maximizing your resources (1-2 mana and some graveyard digging) for greater gains than your opponent (you're getting a 4/5 body for 1-2 mana; how much body will 1-2 mana buy your opponent, Tarmogoyfs aside?). You can thus usually cast 2-3 things in the delve-creature turn, whereas your opponent, for a similar body, is paying more mana and using most of his turn to do it.
The trick with Sassigur (which also more or less explains Angler) is to not get greedy with what you keep in your 'yard. Unless it's your only kill spell in your whole deck, delving out a Doom Blade to cast him faster is usually the correct choice, for example, because planning to activate his ability to pull multiple things out of your current 'yard is absurdly greed and inefficient use. Sometimes, cards just need to be played out, and damn whatever sounds most "efficient"; you can't spend the whole game scheming about slamming down Sassigur and pulling your whole library back into your hand, the person across the table from you is trying to kill you!
Over here, cracking a fetch early, casting a cheap spell, and having a creature hit the 'yard isn't that unusual by T3, making Tas, at the worst. 3-mana 4/5 with upside on T3. If your manabase can support it, that means next turn you untap with a 4/5 beater and can use that ability too, if you so choose (though it's usually best to use it later, the point stands). And beyond T3, he usually ends up being 1 mana in virtually any deck, which is, again, real sweet for keeping up the tempo pressure. So while, yes, he does come down later than T1/T2, his value is in letting you do more on the later turns that you might drop him on, or letting you get ahead on big bodies with a 4/5 on T3 regularly if you play him out quick.