Yeah, topdeck abilities are usually intended to be tamer versions of other cards that just let you search for a card directly, but the timing is definitely key. If you know you're going to be searching before you draw again, it's probably not worthwhile to topdeck a card from your deck or reserve. Prophecies of Christ and Lineage of Christ (the most recent two sets) introduced cards with Star abilities that can be activated when drawn by game rule, so having those take cards directly would give an overpowered card advantage. Instead, many of them were given topdeck abilities so that you could still get the card you want but wouldn't have it right away.
There are also some synergies with topdeck and underdeck abilities that you can take advantage of. Jeshua, the Restorer is a Territory Class Hero that lets you draw a good card that you just topdecked. If you're playing a demon deck, the you can use The Bronze Laver to underdeck demons from your hand and draw and then use The Gates of Hell to put them in your territory. Other cards like Preparing to Return and Seven Years of Plenty let you draw in your upkeep so if you draw a card with a Star ability that lets you topdeck something then you'll have a way to draw it right away.
So while topdecking cards isn't the strongest ability, it can be effective if you time it right, like playing Triumphal Entry to topdeck Son of God right before attacking with King Hiram to draw 1.
