The in-game shop offers petals for stars on a rotating schedule. Because the selection changes, checking the current rotation (or receiving a shop-update notification) prevents you from missing a petal you specifically need.
Stars as currency
Stars are obtained through play, challenges, and other progression systems. High-rarity petals in the shop cost large amounts of stars; Super-tier offerings sit at the extreme end of the price scale. Budgeting stars for a target petal is often more efficient than pure crafting when the success rate is already very low.
Why rotation alerts help
Shop contents are not permanent. If you are saving for a specific high-tier petal, a notification that it has rotated in lets you buy before it disappears again. The same principle applies to any limited-time or rotating content the game introduces.
Combining shop and crafting
Many players mix both approaches: farm and craft the bulk of their progression, then use the shop to fill gaps or to obtain a petal that would be extremely expensive to craft from scratch. Tracking both rare mob spawns and shop rotations covers the two main external sources of high-value petals.