You are hiking up the side of a mountain or blazing through a local event. Carrying a full backpack isn't likely to be helpful when you only need a few items for the day. Using a cross-body shoulder strap packs are a great way to minimize weight while still holding the items you need.
While they don't offer the same level of carrying capacity you can certainly put phones, wallet, water, first aid, and basic rations in one of these packs. Much more effective than the traditional "fanny pack".
Other uses include biking and walking. When I go sailing I often like to bike over to the race marina from my boat and I will used one of these packs because it fits my gloves, hat, water and food for the day. $18.99 Free Shipping