In Animal Parade, there usually are two ways to obtain any item needed to complete a Quest or even to obtain a 'Most Favourite Gift' for an individual. One is through your own efforts either by farming, raising Animals, mining, fishing or cooking. The other is by purchasing the item from a local Shop or Festival Stall. Almost every item needed for any Quest will appear on a Shop menu at some point. Some require the highest upgrade, but they DO appear.
If you are looking for Good Corn, for example, you either can grow it yourself in Summer or purchase it from the Marimba Farm when it is upgraded to a Level 3 Shop. Likewise, Shining Honeydew will appear on the Menu at Level 4.
The Witch Princess loves Blackberry Cocktails. You can forage for Blackberries and pop two in an Aging Pot to make Blackberry Cocktail or you can purchase a Blackberry Cocktail from the Brass Bar when it is upgraded to Level 4.
These are only a couple of examples among many. Basically, I suspect that the reason why Shops sell all the vital items for Quests is to encourage players to SHIP their items rather than hoarding them for future use. Shipping is fundamental to Animal Parade. Unlike other games where Farming Titles are progressive, Titles in Animal Parade are awarded at the end of each season and endure only for a single season. Your Farming Titles are based solely on Shipping Totals in various categories. If you hoard items (like me), you will not be able to earn a decent Title at the end of any Season. You actually MUST obtain a Hero Title in one category in order to meet the Harvest King to complete the Quest of the Five Bells, restoring the Goddess Tree to life.
You can hoard items until the last day of any Season, however, without affecting your ability to earn a Title. You actually can save your game on the 28th day of a Season and then experiment by shipping various quantities, reloading and adding to the shipping container until you win the 'Hero' Title.
There is a special award for obtaining the Ultimate Title in EVERY category at the end of a season. The easiest way to win that would be by hoarding items in each category until you have a sufficient total in each to obtain the Ultimate Title across the board, as it were.
It always is less expensive to grow or obtain your own items rather than purchasing them from a Shop but saving money never is the primary goal in any Harvest Moon or Rune Factory game. You need to spend money, in any case, in order to make progress. In Animal Parade, often you will find that purchasing an item is a better strategy than waiting to obtain it through your own efforts.
Where Festival Contests are concerned, purchasing items from shops can allow you to win a Contest you otherwise might not win.