I agree with you and i have the same issue, what others do not get it is we require some sort of tagging our transaction.
Num field should be able to be used for that. I also agree with others about the ease of data entry using desc column as gnucash remembers and autofills amount, account, etc. Nevertheless, you can use Num field to be used for any way you please like a hashtag. One good use of num tag is to order things when it is the same date for multiple transactions.. enterting 1, 2, 3 will sort it in that order when the dates are same.
Here are some excerpt from the book: Gnucash 2.4 Small Business
Accounting
Transaction number
You can use the Num field for Transaction Number,
Check Number, Receipt Number, or another similar purpose. You can
enter alpha-numeric text, as needed. Using the Num field for sorting
transactions You can use the Num field to enter a serial number for
all transactions in an account within a given date. By default, the
account register sorts the transactions by date and, within a given
date, by the Num filed. This is one possible way of using the Num
field
Split action
The Action field is only available when you expand the transaction
using the "Split" button or in Auto-split Ledger and Transaction
Journal views. The action field has a drop-down list of suggested
entries for different account types. However, you need not be
restricted to this list. If the list doesn't meet your needs, go ahead
and enter other text as desired.
Example of using Action field
For example, you might want to keep track of Auto:Gas expenses marking
Action as 'Local', 'Richmond', and 'Other'. This will help you filter
transactions and create reports for gas expenses incurred on trips to
Richmond. If the gas expenses under this category exceed a particular
threshold, you intend to consider moving your offices to Richmond
Please note you cannot search transactions by Num or Action.
However you can use Find in edit menu.
Also most if not all of the reports, you cannot filter by Num or Action.
Only by Desc, Memo and Notes field, you can do any of the filters.
The 'NUM' field in GnuCash is for the Check # or other numeric ID associated with the transaction.