The gemara says that if one prunes and needs the wood, he is in violation of both planting and harvesting. Tosafos explains that the implication of the gemara is that if one doesn't need the wood, he is only in violation of planting, not harvesting. This would seemingly only be true according to Rav Shimon who holds that one is not chayev for a melacha sh'ein tzricha l'gufah, therefore although one is technically violating both planting and harvesting, since he doesn't need the wood, he isn't chayev for harvesting. But, Tosafos says that the gemara implies that the requirement of צריך לעצים, needing the wood, is necessary even according to R. Yehuda. The rationale is that the act of pruning is not considered harvesting at all unless his intent compliments the act, meaning that he is doing it for the wood.
Tosafos enlightens us to a fascinating concept. Sometimes the melacha is defined simply by the action that one is doing. For example, if one digs a hole they are doing a melacha of plowing, even if they don't need the hole. Whether they are chayev or not will depend on the machlokes between R. Yehuda and R. Shimon, but either way we consider them to have violated the melacha. However, some melachos are DEFINED by the intent one has when doing them. If one were to cut branches without the intent to use the wood, the act is just defined as an act of pruning which is a melacha of planting, not harvesting. Tosafos proves this point from קורע על מנת לתפור and from מוחק על מנת לכתוב. Tosafos holds that for tearing and erasing the requirement for it to be for the purpose of sewing and writing respectively, is not merely a condition in being a מלאכה הצריכה לגופה or avoiding the problem of it otherwise being a destructive act. Rather, Tosafos holds that the melacha is defined as tearing for the purpose of sewing and erasing for the purpose of writing. R. Akiva Eiger (gilyon hashas) points this out. Even if the act of tearing or erasing would be done for a constructive purpose, just not for the specific purposes of sewing and writing, it would not be a violation of the melacha. This would helps explain the Rosh (siman 6) who writes that untying is only a violation if done for the purpose of tying. The Tiferes Shmuel asks that since untying something isn't a destructive act, why do we insist on untying being solely for the purpose of tying. Even if not for the purpose of tying he should be chayev since it is a constructive act? Based on Tosafos we can suggest that the requirement of it being for the purpose of tying is not to make the act productive, rather it is the very definition of the melacha similar to tearing and erasing.
The difficulty with Tosafos is that they define the act of tearing to be "tearing for the purpose of sewing", implying that for any other constructive purpose, one would not be chayev. R. Akiva Eiger points out that this contradicts the gemara 105b that says if one tears his clothing as kriah for a relative or tears something to alleviate his anger, since it is constructive they are chayev for tearing. The gemara strongly implies that tearing need not be for the purpose of sewing so long as it is for a constructive purpose, contradicting the assumption of Tosafos?
R. Elazar Meir Horowitz, without citing R. Akiva Eiger, addresses his question. Tosafos doesn't mean to say that the only time one is chayev for tearing is when it is done for the purpose of sewing. So long as it is done for some other constructive purpose, they are chayev. However, when there is no other constructive purpose other than to resew, we insist that he is actually doing it for the purpose of resewing, not merely that it be fit to resew. We see that even though to avoid the mekalkel issue it should be sufficient that it is torn in a way where it can be resewn, he is only chayev when he is doing it for the purpose of resewing, because that becomes the definition of the melacha.