It depends on how much lighter weight or a more compact size is important to you.
Typically any pack 4Ah or larger is going to use two parallel series of cells which (all else equal) doubles the current capability of the pack, and likewise above about 7Ah you'll have triple parallel cells. This is not a rule as some manufacturers venture into use of cells other than 18650 size with varying per cell capacity, but it's true more often than not, a general range which determines how many series of cells are in parallel (research it for the tools you are interested in).
With smaller tools this may not matter, except that as you get lower and lower in pack voltage as it drains, the tool will still have lower and lower performance, though this impacts brushed motor tools far more than brushless.
It means a longer battery lifespan. If you have a double capacity pack you only have to charge it half as often to get the same work done, so you're buying new packs less often, though practically speaking there is also Li-Ion shelf life meaning you can't extend that indefinitely, may not get over a decade out of a pack before it gets weak even if you barely use it.
The only time I would get the smaller capacity packs (over at least medium capacity is if the intended purpose is a trade where you do a lot of overhead work and the weight savings saves your arms and back. You can get multiple smaller packs and swap them as needed, but unless you need a break anyway, it's better to not have to go back to the charger to do a swap... if you're too tethered to the charger location, did you really need cordless?
Depends on the tool, you can own BOTH higher and lower capacity packs. Some things like leaf blowers or saws, really need the larger packs to work more than an hour (if that) continuously, won't run close to all day on a 5Ah pack. Even a 1/2" drill or impact wrench won't go a couple hours continuously on 5Ah if you figure a load running at 50% duty cycle. Li-Ion packs are great for cordless tools but they aren't THAT energy dense.
Batteries are sold separately so you can go either way and end up with the mix of capacities that suits your use, once you've hand some hands-on time to determine which you need more of... but if you plan on only buying a set and using the batteries that come with it, do get the higher capacity batteries.