Dr. Gero is a walking contradiction in Dragon Ball Z. From his android creation and naming methods to how he even turned himself into an android, the man is an enigma. Cell, Android 17 and Android 18 are Dr. Gero's most famous creations, as well as the most complicated. It's not entirely clear why he created Cell to absorb two androids that were each strong enough to kill Goku on their own, had they only managed to find him sooner. If only 17 hadn't been so quick to kill Dr. Gero, he may have been able to explain why everything he makes is so confusing.
There are many odd choices in Dr. Gero's designs, such as androids with accents or large, bulky frames. Installing pain receptors was also a strange, seemingly self-sabotaging move. Even Cell's creation is unorthodox, considering he's the ultimate creation yet designed to be imperfect and requiring years of incubating. Dr. Gero's design was advanced enough to allow complete regeneration and retention of cognitive function from a single cell, but only on the condition of outside improvement by way of absorbing a pair of seemingly unrelated-to-Cell androids.
It could be as simple as the notion that Cell's full potential for power wasn't something that Dr. Gero could imbue him with. However, that doesn't explain why Cell specifically needs 17 and 18 to transform and grow stronger. By the time Cell nabs 18, Vegeta and Trunks are far superior to her in strength, so it seems logical that absorbing one of them would be a much greater boost than absorbing 18 could possibly be. Nonetheless, Cell doesn't show interest in absorbing any of the Z Fighters, claiming he already has enough of them in his DNA, so perhaps doing so would be pointless.
Cell was always Dr. Gero's endgame, so it stands to reason that 17 and 18 were created for the singular purpose of granting Cell greater strength than Gero could give him. By finishing the androids first, Dr. Gero would have something to offer Cell because, as 17 demonstrated, Dr. Gero's creations aren't inherently loyal to him. Who's to say if Gero would have ever activated 17 and 18 were he not being pursued by the Z Fighters? The idea that the androids were designed to be absorbed would make it clearer as to why Cell specifically requires the androids to transform, rather than just any source of immense power.
Considering that Dr. Gero removed his own brain and put it in an android body without any help (or dying), he's clearly a very capable scientist. It therefore seems unlikely he would have created Cell the way he did if he were able to make him perfect from the start. Maybe Dr. Gero designed Cell this way in order to make Cell easier to control, but it's more likely that Cell's imperfect form and absorbing ability were simply the best he could do. Then again, Dr. Gero is a madman, so any and all explanations for Cell's design remain on the table.
There's no arguing that Cell, despite his unorthodox design, is Dr. Gero's crowning achievement and the perfect tool for revenge. Goku's been living in Gero's head rent-free since Dragon Ball, and the mad scientist decides to use his one strength that Goku lacks: intelligence. In the end, Dr. Gero may have simply decided that an overly complicated plan involving androids absorbing other androids would be more than Goku could comprehend, and his nemesis would be defeated before he could even ask what an android is.