Dragon Ball Z has only ever had one rule: wait for Goku. Even Piccolo, one of the franchise's most iconic characters, is not immune from this narrative. In the Cell Saga, the rejuvenated Namekian attempts to take matters into his own hands. As a result, a character known for his intelligence is forced to make several critical errors in judgment so the story can still make its way to Goku and Gohan saving the day.
In order to get stronger in a hurry, Piccolo heads to the Lookout to use Namekian Fusion with his counterpart, Kami. The resulting power from this union makes Piccolo, for the moment, the strongest of all the Z Warriors -- including Goku and Vegeta. Before paying the androids a visit, however, Piccolo encounters a new threat in the form of Imperfect Cell.
In their brief scuffle, it's clear that the fused Namekian is the superior fighter. Imperfect Cell, not overly intelligent to say the least, is kind enough to explain exactly what his plans are to a wounded Piccolo. Somehow, Cell is surprised to learn that Piccolo can regenerate even though he himself contains the Namek's DNA, having used the Special Beam Cannon against its creator already.
Piccolo, having feigned weakness to get Cell talking, regrows his arm while backup arrives in the form of Trunks and Krillin. Sadly this is where it all falls apart, as they all just kind of stand there staring at Cell instead of immediately ganging up on him. Piccolo even warns them about Cell's tail but doesn't think to have the time-traveling Saiyan use his sword to cut it off to buy them some time in battle.
Cue the most flabbergasting technique in all of Dragon Ball Z: Tien's Solar Flare, which unleashes a debilitating flash of light. Cell slowly rises into the sky, strikes a very distinctive pose they are all familiar with, and literally screams the name of the attack -- yet in all of that time, Piccolo doesn't think to rush him or simply close his eyes.
Ultimately, Cell escapes because the Z Fighters never learned not to look directly into the sun. Unlike the Androids, he has a very distinct kind of power that can be sensed and therefore tracked. Displaying an out-of-character lack of critical thinking skills, Piccolo decides he will destroy the Androids to keep Cell from absorbing them. This doesn’t make sense for a lot of reasons.
Piccolo already knows he is stronger than Cell, but it remains to be seen if he is strong enough to destroy Androids 17 and 18 -- as well as 16, should he interfere. Additionally, the Namekian has no idea where the trio is or what they may be doing. Cell, on the other hand, is pretty openly going from town to town absorbing humans and getting stronger.
Meanwhile, Androids 17 and 18 are just messing around and not causing much mayhem at all. As long as they aren't terrorizing the planet, it would have been perfectly reasonable to put destroying them on the back burner. Piccolo's choice to leave Cell to his own devices and challenge the Androids was the worst thing he could have done.
It is important to note that Cell cannot sense the Androids either. There's no telling how long it would have taken him to find them on his own. However, he can sense Piccolo's power -- so when the Z warrior engages Android 17 in battle, the dinner bell has been rung. When Cell arrives on the battlefield, it's immediately clear he has powered up significantly in the time since Piccolo let him go.
Piccolo's terrible decision-making results in his own near-death as well as the absorption of Android 17. Had he exhibited a little more of the intelligence he's known for, the Cell Saga could have ended before it truly began. Rather than this being Dragon Ball Z's story of Piccolo saving the world, it became the last time he was ever relevant in a battle against the villain.