In a situation where two people together do a melacha on shabbos, but one is capable of doing the entire thing on his own and the other is not, the gemara says that the one who can do it on his own is chayev and the other is exempt because מסייע אין בו ממש. If the אינו יכול who could not do it on his own would be considered significan (מסייע יש בו ממש) it is not clear from the gemara what would happen. Tosafos writes that they would both be chayev because it would be like אינו יכול ואינו יכול where they are both chayev (at least according to r. meir and r. yehuda but the rashba understands that even r. shimon would agree that they would both be chayev). However, Rashi (d.h. ein bo) implies that if the אינו יכול is considered significant, it would make it like זה יכול וזה יכול and they would both be patur (acc. to R. Yehuda who we pasken like). Nevertheless, the gemara concludes and proves from various places that we pasken מסייע אין בו ממש therefore the יכול is chayev and the אינו יכול is patur.
Tosafos (d.h. Amar) raises a question from the gemara in Beitza 22a in the context of a goy putting eye ointment in for a jew on yom tov and the jew assists by opening and closing his eye which is a Rabbinic prohibition of refuah. In that context both R. Zevid and Rav Ashi say מסייע יש בו ממש so that even though there is no prohibition on the goy, there is an issur on the jew to assist as it is being done for him. Why in that context do they assume מסייע יש בו ממש and in our context we assume מסייע אין בו ממש?
The first answer of Tosafos is that the gemara here is working off the maskana of the gemara in Beitzah where everyone came around to hold מסייע אין בו ממש. The second answer of Tosafos is that for a melacha d'oraysa we hold מסייע אין בו ממש, but in the gemara Beitza it is speaking about the gezeira d'rabonon of שחיקת סממנים, and in that context rav ashi and rav zevid consider מסייע יש בו ממש. What is the rationale to be more machmir for מסייע by the gezeira of שחיקת סממנים? When it comes to an issur d'oraysa we focus on the ma'aseh issur, therefore when one is merely a מסייע we don't consider his action significant enough to make him chayev. However, when it comes to the issur of refuah which is a rabbinic concern that it will lead to שחיקת סממנים, we don't need to focus on the specific action, rather any involvement that may lead someone to grind the medicine would be included. A מסייע may also be led to grind the medicines, that is why in this context we consider him יש בו ממש.
The Ritva suggests another answer. In a situation where the מסייע ultimately doesn't contribute that much because the result is the same with or without their help, such as in our case where the other person can carry out the item without the מסייע, we say מסייע אין בו ממש. But in the sugya in Beitza, if not for the מסייע opening and shutting his eye, the ointment would not be as effective, therefore we say מסייע יש בו ממש.