Collaboration is the most important thing on any project. A team is a team and no longer a group of people working together, if there is a free flow of information going back and forth amongst the members of the team and not just up and down a functional hierarchy.

Comments

Whenever a new comment is left on any issue it is advertised on the dashboard in the project updates section. That allows to spot them easily and respond to.

Comments to discuss a story before implementing it

Geographically distributed teams can't easily talk to each other as often as they might want to. Talking together requires as well to get everybody to be present at the same time, which might not be possible due to timezone difference or simply interrupts work being done.

Teams that have their own team room and can practice sit together are better off, as they can talk about something anytime they want to. Still the issue of interrupting work being done remains.

Customers, stakeholders should be involved, which is probably the hardest thing to achieve. In the case of corporate software development their primary function is not to be part of a software development team but to run the business that wants to use the new software product. Commenting on issues that are still in the backlog allos them to participate on their own schedule and provide guidance.

Commenting on issues is not a substitute to meet and talk Don't rely on comments only. If your team can meet to talk, seize that opportunity as often as you can. You may use comments to document a decision that has taken by those who talked about an issue in a meeting.

Comments to get answers and share information

Once issues have been moved into an iteration to work on them comments are useful to coordinate amongst team members and to ask questions to stakeholders.