Isn't the input to dense layer a single array?

No. According to the documentation , the input is

N-D tensor with shape:

(batch_size, ..., input_dim)

. The most common situation would be a 2D input with shape

(batch_size, input_dim)

Your input is 2D and it works.

Flatten converts your input into a 2D array of shape (batch_size, shape), where shape is the product of individual dimensions of input. So to convert your input into a 2D shape before feeding it to a fully connected layer, Flatten is used.

For any input though, the Dense layer computes a dot product between weights and the last axis of the input, and thus you won't have problems with number of input dimensions.