Bugs arrive through emails, chat messages, testers' notes and crashes on customers' phones. Unless they are collected in one searchable place, they are forgotten. It also serves as a knowledge base: someone may already have found the cause of your strange glitch. Spolsky's company went on to sell one, FogBugz.
A useful report records the steps to reproduce, the expected and actual results, the environment (browser, version, operating system, screen size) and a screenshot. On GitHub 29 an issue form, a YAML file in .github/ISSUE_TEMPLATE/, makes those fields required. GitHub Issues and GitLab 377 Issues live next to the code; commercial trackers such as Jira 619 and Linear add planning and reporting. Error-monitoring services such as Sentry 92 file bugs you never saw, capturing JavaScript exceptions from real users' browsers with stack traces and grouping repeats into one issue.