Many people use Planyway to track their projects. Viewing week numbers allows you to track your progress on projects from week to week, instead of by month. Since months often start and end on different days and have a different number of days per month, setting up a calendar by month for longer projects can be a nightmare. Planning your projects by week allows you to have a calendar that is accurate and unvarying.
To enable week numbers, just click Settings → Show week numbers.