Tosafos questions whether the language of חובה has an implication of being a practical necessity, but not a mitzvah and therefore not worthy of a bracha. However, Tosafos writes that when we say that lighting candles for shabbos is a חובה, we don't mean to the exclusion of a mitzvah and therefore a bracha may be recited.
Tosafos then says that there is another reason that some hold not to make a bracha on shabbos candles. Since if it were lit from before shabbos one would not be required to extinguish the flame and relight, therefore even when they do light, no bracha is made. Tosafos rejects that rationale, even if the assumption that one wouldn't have to extinguish and relight was correct, from the bracha on kisuy ha'dam and on bris mila. By kisuy ha'dam the gemara says that if the wind covered over the blood, he would be exempt from the mitzvah, yet when he does the mitzvah he makes a bracha. Similarly, if one would be born circumcised there is an opinion that he doesn't even require hatafas dam, yet when one does a bris they must make a bracha. Here too, although if the candle was burning from before, one would not need to do anything, however, when there is not candle burning they are required to light with a bracha. At the very end Tosafos rejects even the premise and holds that a candle that was burning from before, must be relight for the sake of shabbos [Tosafos proves this from the gemara 23b which says that one cannot light too early as rashi explains because it won't be recognizable that it is for shabbos - clearly a candle burning from before isn't sufficient. However, it could be that if one is going to do the mitzvah, they must light in the proper time, because lighting early isn't a fulfillment of the mitzvah rather an avoidance of the mitzvah. But if one had a preexisting candle burning, they would have no obligation to light at all].
The machlokes between the two opinions of Tosafos seems to be how to define the nature of the mitzvah to light candles. Is the mitzvah to do a מעשה הדלקה to ensure that there is light for shabbos, but the mitzvah is still the מעשה הדלקה, or perhaps the nature of the mitzvah is simply to ensure that there is light for shabbos and the act of lighting isn't an act of a mitzah? The first opinion seems to hold that the fact that a light burning from before is sufficient indicates that there is no mitzvah in the act of lighting, therefore even when one lights, no bracha is said. Whereas the second opinion holds that although the goal is to provide light so that if one already had a candle there is nothing to do, nonetheless, without a candle they are obligated to actually light and therefore a bracha can be recited.