On the edit page for each attribute (Admin > Attributes > Attribute [edit]) you can decide if a particular attribute should be made mandatory at the point of upload.

There are two tick boxes for this, ‘Mandatory’ and ‘Mandatory on Bulk Upload’ (see figure 1).

Figure 1 – Edit attribute

An example of why both of these settings exist are as follows:
You may want to make the ‘Description’ metadata field mandatory when someone is uploading a single asset. However, if someone is bulk uploading 30 assets, the chance that they all have the same ‘description’ information is unlikely. So the two options allow you to be more flexible.
Note: After bulk upload, the uploader usually Batch updates the files they have uploaded. This lets them step through each asset, inputting metadata that is specific to each. At this point, the first ‘Mandatory’ setting comes back into play.

In summary, when you are uploading single assets, you may want to have mandatory fields set that do not make sense on Bulk Upload.