What's this block aside the part to be build?

It is called a Purge Block. When the Palette transitions from one color to another there is a period in which the colors are mixed and that needs to be purged.

Here is a great video explaining the ins and outs of purge blocks - https://youtu.be/lR_c-zxW2D8

It does not matter if you are using a Palette, Enraged Rabbit, or Prusa’s MMU you will get a purge block.

Adding to @petalys, explanation, you can control the placement and size of the tower in the following ways:

• In the toolbox, the scale tool can be used to change the dimensions of the block, but not the volume. You can change it to a thinner block to have more space on the bed.
• The layer height entered will also slightly affect the tower size. 0.25-0.3mm are good to start with.
• The transition length under Project Settings > Transition will allow more control over how much filament is purged on the tower. Variable transitioning will allow less to be purged for light colors, and more can be used for the darker or stronger colors.