There is a variety of reasons why they need a second (or first) striker besides Joselu. First of all they need quality backup who doesn't need too long to adapt or even better, who is readily available to jump in if needed and perform well. A club with the goals that Real Madrid has can't afford to be without a striker at any phase in the season. Bayern had so much trouble because they thought they could improvise and fix the striker issue with some of the players who have good striking qualities but are actually not real strikers. It didn't work out as they were often left without solutions in the game.
Other reasons can also have an impact when one of the strikers is injured and not having another striker will affect the team, although Real Madrid could try to put several options up front with the players they have. Real Madrid really needs a single striker who can make the attack line sharper and this figure is needed because Ancelotti often relies on typical strikers who attack. I watched Joselu and Brahim Diaz during the friendly match against AC Milan and even though their performance was good their goal-scoring instinct was not strong and there were a few opportunities that they were not able to take advantage of.
Joselu would be the backup if you ask me but Real Madrid has to wait and see what will be possible on the transfer market. The situation is changing on a daily basis. But the second reason is that a team needs competition for any position. A striker who knows that there is nobody else to take his position may be less inclined to play his best. Competition is always good.
And lastly there is never a guarantee that a player can cope with the Real Madrid pressure unless it is Mbappe or someone like Ronaldo. They don't need any time to adapt, but Joselu isn't like that or maybe he proves me wrong.
Every striker definitely wants his position and playing hours not to be replaced by other players because it will affect their performance, but without a backup striker the appearance of the main striker will not be motivated to improve his quality because there is no rival and that is also one of the reasons why Real Madrid needs another single striker, however Ancelotti has his own views regarding the attacking position he has and I believe he has a special format to maximize the front line.
Mbappe is a striker who has experience under pressure and he has also played at the highest level for both club and country, so adapting will probably be much quicker, whereas Joselu may need time to find that level and it will depend on how he performs to reach that level.