The point is that it is an option they have which subtly costs them significantly more money than it advertises. It makes itself appear to be a roughly equal option with a known, easily calculable fee. It can be budgeted, and might for someone working long hours who doesn't have the time to walk however far or travel however long on public transit to get to the grocery store and back (or who simply sees a roughly equal option which saves them a lot of time).

The reality is that the algorithmic prices are always higher than they'd be in store. InstaCart would never lower their prices below what's in the store, algorithmically or otherwise; they would have to foot the difference. Using software hides these price increases, especially when the nature of it means the customer won't be in the store to see the differences in real time. (They could enlist help but why would they even think to do that?)

It seems to be a reasonable option even though their grocery bill will (apparently) be about 7% higher. That could seem the same as prices rising; it's surely noticeable and will make them drop a couple items from their cart but they can still checkout. Their cost would remain consistent over time and with actual price increases while always being that barely significant amount more. All while they believe they know exactly how much their decision is costing them.

Cars breakdown. Bodies become injured. The weather gets bad. Babysitters get ill or quit.

Many things can leave a person temporarily or permanently unable to fetch their own groceries. Many of those things happen to the poor more frequently. They at least have more severe consequences when a person is poor.

For example, a middle class person can just buy a new tire when one goes flat. A poor person may have to walk for a few paychecks.