Credit to Wikipedia https://en.wikipedia.org/wiki/Handloading#Reasons_for_handloading
Economy, increased accuracy, performance, commercial ammunition shortages, and hobby interests are all common motivations for handloading both cartridges and shotshells. Reloading fired cartridge cases can save the shooter money, or provides the shooter with more, and higher quality, ammunition within a given budget. Reloading may not be cost effective for occasional shooters, as it takes time to recoup the cost of the required equipment, but those who shoot on a regular basis will see benefit as the brass cartridge case or shotgun shell hull (the most expensive components) can be reused many times (with proper maintenance). Besides economy, the ability to customize the performance of ammunition is a common goal. Hunters may desire cartridges with specialized bullets or specific performance as regards bullet and velocity. Target shooters seek the best achievable accuracy, as well as the best shot-to-shot consistency. Shotgunning enthusiasts can make specialty rounds not available in commercial inventories at any price. Many handloaders also customize their cartridges and shells to their specific firearms, usually in pursuit of accuracy: they can assemble precision ammunition using cartridge cases that have been fire formed in the chamber of a specific firearm.
Handloaders also have the flexibility to make reduced-power rounds for hunting rifles, such as handloading to an equivalent of a milder-recoiling round to encourage recoil-averse hunters to become proficient with a full-power one. Rather than purchasing a special purpose rifle, which many novice hunters would outgrow within a few hunting seasons, a single rifle can be used with special handloaded rounds until such time more powerful rounds are desired and become appropriate. This use of specialized handloading techniques often provides significant cost savings, especially when a hunter in a family already has a full-power rifle and a new hunter in the family wishes to learn the sport. This technique also enables hunters to use the same rifle and caliber to hunt a wider variety of game.
Collectors of obsolete firearms who want to shoot those guns often must handload because appropriate cartridges or shotshells are no longer commercially produced. Handloaders can also create cartridges for which no commercial equivalent exists - wildcat cartridges. As with any hobby, the pure enjoyment of the reloading process may be the most important benefit.
Recurring shortages of commercial ammunition are also reasons to reload cartridges and shotshells. When commercial supplies dry up, and store-bought ammunition is not available at any price, having the ability to reload one's own cartridges and shotshells economically provides an ability to continue shooting despite shortages.
There are three aspects to ballistics: internal ballistics, external ballistics, and terminal ballistics. Internal ballistics refers to things that happen inside the firearm during and after firing but before the bullet leaves the muzzle. The handloading process can realize increased accuracy and precision through improved consistency of manufacture, by selecting the optimal bullet weight and design, and tailoring bullet velocity to the purpose. Each cartridge reloaded can have each component carefully matched to the rest of the cartridges in the batch. Brass cases can be matched by volume, weight, and concentricity, bullets by weight and design, powder charges by weight, type, case filling (amount of total usable case capacity filled by charge), and packing scheme (characteristics of granule packing).
In addition to these critical items, the equipment used to assemble the cartridge also has an effect on its uniformity/consistency and optimal shape/size; dies used to size the cartridges can be matched to the chamber of a given gun. Modern handloading equipment enables a firearm owner to tailor fresh ammunition to a specific firearm, and to precisely measured tolerances far improving the comparatively wide tolerances within which commercial ammunition manufacturers must operate. Where the most extreme accuracy is demanded, such as in rifle benchrest shooting, handloading is a fundamental prerequisite for success.
Insurgency and resistance groups, as well as military partisans, might also have need to handload cartridges and shotshells due to unavailability or scarcity of factory-produced commercial ammunition within particular jurisdictions, or under certain circumstances. Low quality of available factory ammunition, even without scarcity of ammunition, can also lead to the need for widespread fabrication of handloaded ammunition.