In dedicated platforms .40S&W is a great upgrade from 9mm. For those people who don't like 9mm very much and for whom .45auto is a bit more than they like to shoot (recoil-wise) forty very nicely bridges those two calibers.
For me, the little 9mm rounds are fiddly to handle and no fun to reload. And while 9mm can be an effective defense load, it does fall short of .40S&W, just as it falls short of .45auto. The many claims that 9mm is the EQUAL of .40 and .45 are false claims. No two calibers are "equal", as they each have qualities that differentiate them.
Even using the
very imperfect model of gelatin block testing (which does not resemble the human body very well at all - it has NO BONES, for one difference) the different calibers behave differently - not identically.
Heavier weight projectiles of larger diameter can bust through obstacles (like heavy bone) more easily than light weight projectiles. This is one distinction that differentiates .45 from .40 and .40 from 9mm and 9mm from .380 and .380 from .32.
If 9mm is "just as effective" as .40S&W, then .40 is just as effective as .45. And .380 is just as effective as 9mm. And .32auto is just as effective as .380. No, it isn't so.
Please, 9mm fanboys need not get defensive, nor feel threatened by calibers such as .40S&W nor by .357Sig, .45auto, or 10mm, etc. But for some reason, many who have ingested the 9mm cool-aid deeply behave very defensively (sometimes offensively) when discussions of .40S&W and .357Sig and .45auto take place. They don't seem quite as defensive in the face of .45auto discussions, but seem to take great defense/offense when it comes to .40S&W. I guess forty gets just a little too close to 9mm (relatively high capacity) for their comfort. But no reason for that, as they're both good calibers, each with their own particular benefits.
All calibers are excellent calibers in their own ways, but none of them are "identical" to any of the others. They each offer a particular set of qualities that have different benefits making each better suited to different platforms and applications.
Personally, I don't like 9mm very much, except when I need to step down to small platforms like the PM9 or P938. I enjoy .40S&W much more when using mid-size and full-size platforms. But I don't deny that others may enjoy 9mm for what they carry and what they shoot.
Generally speaking, I find the smaller calibers (like 9mm) more suitable and enjoyable than the larger calibers when shooting small pistols. And when shooting larger pistols, I like the larger calibers better.