It is a small line in a comparison table and it changes the total cost of ownership more than most features do. If your accountant or auditor needs a seat and the vendor charges for it, that is another user price every month for the life of the system.
Who gives it away
Sage Accounting includes accountant access on every plan at no cost, with its own permissions, and the accountant can work in the file at the same time as your bookkeeper. Zoho Books includes one accountant seat even on the free plan.
Xero takes a different and arguably better approach: every plan has unlimited users. There is no accountant seat because there is no seat limit at all. For a business with a bookkeeper, an owner, an accountant and an auditor, that is four seats that cost nothing.
Who charges
QuickBooks Online counts your accountant against the plan user limit on the lower tiers, so a Simple Start subscription with one user means either you or your accountant, not both. In practice that pushes businesses onto Essentials.
Desktop products are their own category. Sage 50cloud Pastel licenses per concurrent user, so an accountant working in the file occupies a seat while they are in it. Practices generally handle this by taking a backup and working on a copy, which is workable and means you are not looking at live data together.
The related question: can they get the data out
More important than the seat, honestly. At year end your accountant needs a trial balance, a general ledger detail, an age analysis and the supporting documentation. Ask two things during evaluation.
First, can the accountant export a full general ledger detail to a file they can work in, without the vendor's help. Second, are transaction attachments included in an export, or do they live only in the interface. The second question catches people out at audit time, when three hundred supplier invoices turn out to be viewable but not extractable in bulk.
What we would recommend
Ask your accountant which systems they hold current certification in before you shortlist. A practice working in a system they know charges less and finds more. If they have no preference, weight unlimited or free accountant access meaningfully, because over three years it is a real number.
And if a vendor cannot give you a straight answer about bulk data export, treat that as information about the relationship you are entering into rather than as a technical detail.