Even a small garden shed is heavy and will require a hard, flat surface on which to stand. If you have uneven flagstones or soft ground, given time, it will begin to settle on the foundation, leading to premature deterioration of the shed or garden building.
The door and window frames will usually drop out of alignment, making it difficult to open or close them, rain may enter the shed itself in or it and may even put undue stress on other parts of the structure, leading to cracking or breaking.
A concrete base provides a consistently even and solid surface which will keep even the largest and heaviest garden building stable and correctly aligned. Crane Garden Buildings are experts in building garden sheds and summerhouses and have written an article about why you should ideally use a concrete base.