In General Elections, Voting For Unopposed Candidates Sometimes Matters
In very small municipalities and special districts, an unopposed candidate typically needs at least one vote to be elected, so your vote could make that difference if almost no one else ends up voting.
In cases where, contrary to your question, there is a write in possibility and your vote for an unopposed candidate sets the number of write in candidate votes needed to defeat the otherwise unopposed candidate.
Some states also dissolve small political entities as a matter of law, or commence a process to determine if they should continue to exist, if not enough votes have been cast in multiple recent elections for its offices.
Finally, it isn't uncommon in a general election, for the number of votes cast for an unopposed candidate to factor into a formula for the number of signatures required on recall petition, or for a citizen's initiative proposal.
For example, in Colorado, "The signature threshold to place a recall question on the ballot for a state or county elected official is 25% of the total votes cast at the last preceding general election for that office."
Similarly, the state secretary of state position is not infrequently uncontested, but:
In Colorado, the number of signatures required for a successful
petition is equal to 5 percent of the total number of votes cast for
the office of Colorado secretary of state in the preceding general
election. The secretary of state is elected every four years.
Elections were held for Colorado secretary of state in 2018. The same
total number of signatures is required for constitutional amendments,
statutes, and referendums.
Sometimes local governments also have initiative thresholds that depend upon the number of votes cast for an office that may be unopposed.
In Primary Elections, Casting A Ballot Matters More Than Casting A Vote For A Particular Unopposed Candidate
In a primary election, in a jurisdiction that isn't entirely moribund, none of these factors are likely to matter.
Casting a ballot at all, causes a public record that you voted to be produced which often prevents your voter registration from being classified as inactive, and also signals to candidate and political party campaigns in future races that you are an active voter in a particular location who is sufficiently politically engaged to vote, even in a primary which may be uncontested, making you a target for volunteer and campaign contribution calls, but also a lower priority for get out the vote messages.
what is the reason for listing that seat at all since it is unopposed? Is it just a courtesy to the candidate? Is there a legal requirement? Just logistics of the balloting process?
The unopposed candidate is on the ballot at all, mostly for informational purposes as a well paced campaign ad for that candidate in anticipation of the general election. This is mandated by state election law.
Some jurisdictions cancel primary elections entirely if there are no opposed candidates for any office. Fewer jurisdictions still only list contested races, but this is the exception, and not the rule.
But, to finally answer the actual question asked, the considerations that go into not voting for a particular unopposed candidate on a ballot that is going to be cast otherwise are mostly symbolic:
Voting for a candidate shows some sign, however trivial, of support for the unopposed candidate and will be viewed by the candidate as part of their base of support in the general election for planning purposes.
In contrast, an unopposed primary candidate who receives far less support than other candidates on the ballot will typically be viewed as unpopular with the base and a weak candidate, with the implicit signal that by not casting a vote for the unopposed candidate that you are so unimpressed with the candidate (or so ignorant of the candidate) that you aren't willing to do so. It isn't a loud or strong signal, but collectively, it can send a message too.