For me it was multiple reasons.
I wanted longer battery life than what the cigalikes or Volt X2's (ego-sized battery) could provide. With an 18650 mod battery I can go 1-2 days without a battery change. Also, it's far easier to carry extra mod batteries than another ego battery or multiple cigalike batteries. If I so desire, I can go out for the entire day with a mod topped with a large cartotank, and carry an extra battery in a plastic case in my pants pocket (in the off chance that I would need it).
Mods can carry larger juice devices (up to 7ml cartomizer tanks) than an ego or cigalike.
Another reason is a matter of philosophical and economical preference. The cigalikes and ego-type batteries must be considered ultimately as disposable devices. The ego's are only rated for some 250 charges. When the battery dies, or performance declines because of age, the whole unit needs to be replaced. These devices are also not as durable as a mod. Most mods can take a lot more physical hard use/abuse. Many mods can be repaired by the manufacturer should a part break or fail. Cigalikes and egos? Nope, you need a new one.
On the other side of the coin, mods are built to be extremely durable, generally of thick aluminum or steel, with a life expectancy of years compared to only months. Their fit and finish is of a much higher quality. They generally have a much nicer and durable power button. More vaping attachments are manufactured for mods to offer a larger variety to customize your vaping experience.
When a battery is spent on a mod, you simply replace the battery. The batteries that mods use have a longer use between charges and have an overall longer life expectancy of up to an entire year. Less than $10 a piece, they are considerably less expensive than replacing an entire ego.
Several mods have options to add extension sleeves or extender endcaps to allow use of different sized batteries or to add a Kick (micro-processor with a boost circuit to allow variable wattage).
The downside to mods is because of their larger size, many are not particularly pocket-worthy, and are decidely heavier. However, belt holsters or leather lanyards like Serenity Gear makes are available. Most mods are able to easily fit in the ego traveling cases with room for extra batteries and vaping attachments.
Few people will mistaken a mod for a cigarette-looking device, if that is a concern.


Two mods in an ego-style case with x-tra batteries and tanks. Smaller mod in a leather Serenity Gear lanyard. Serenity Gear belt holsters.
Click the link below in my sig to view some of my mods.