Why do we need to concatenate in a U-Net?
These types of connections between layers are called skip connections, searching for this will give you a way to find more in-depth information. Broadly speaking, there are two advantages to using skip connections in this case:

They allow gradients to more freely flow through the model, mitigating the issue of vanishing gradients
They allow features from the encoder side of the network to the decoder side of the network, adding extra information that might be lost because of the downsampling on the encoder side of the network.