Veteran's Powerblade makes no sense whatsoever. In fact, I'm 95% certain it's a misprint.
Here's the argument:
1- its value is laughably bad. Best case scenario, 4 mana gets you +2/+0 on a critter. Worst case scenario, it's a useless brick. In the middle ground, 5 mana gets you +2/+0 on a critter. The best case scenario isn't close to viable in any format, and wasn't viable in any format (that I know of), basically ever.
2- its bonus is double conditional and extremely limiting. You have to be running white, running a solider(s), and can only get the bonus on soldiers. So, your reward for limiting yourself as stated in to save 1 mana.
3- it's a (much) worse version of other comparable cards, either with or without its bonus. Powerblade came out in 2022 in The Brother's War. Compare it's best case scenario to Bonesplitter, which does exactly the same thing, but for 2 mana less up front, and unconditionally. Bonesplitter came out in 2003 in Mirrodin, and is one of the most cost effective agro equipments in its preferred formats. Or compare Powerblade's regular condition to Sword of Fire and Ice, which, for the exact same cost, grants an additional +0/+2, double protection (one color of which is infamous for removal), and grants two FANTASTIC trigger effects when the equipped creature hits opponent. Sword of Fire and Ice came out in 2004 in Darksteel, and is a staple in almost most formats that can play it, especially when in combo with Stoneforge Mystic.
4- it's a MUCH worse version of other support cards for the same tribe. Veteran's Armaments is a better card than Powerblade in every conceivable way. It costs less to field, it's not color conditional, its tribal reduction condition is more cost effective (4 mana total vs 4 mana total (hard equip) or 2 mana total), it's guaranteed to have a minimum of comparable bonus (+2/+0 vs +1/+1) with a far more likely greater bonus that fits perfectly into the critter-heavy soldier build for which it was designed. Powerblade is an inferior card in all respects. Armaments came out in 2008 in Morningtide.
5- power creep is so rampant in MTG that format-breaking cards are regularly released in modern formats in ever-increasing amounts. What was good 10 years ago is laughable compared to the modern meta, much less comparing cards from 2003/2004/2008 to cards from 2022.
So, unless MTG is now creating bad cards specifically for its lower formats while simultaneously making format-breaking cards that represent disgusting power creep, I'm forced to conclude that there was some sort of mistake made with Veteran's Powerblade.
In reality, the card should either reduce the equip cost to {0} when equipping soldiers or give +2/+1 as it already is. Neither change would make the card playable in any higher format, or make it more playable than other soldier equipment, but it would be potentially viable in lower formats as a monetarily cheap option for soldier builds. A good version of the card wouldn't be a 3-drop at all, or would have triggers like the swords of X and Y (although obviously not to that extent).
So, that's my spiel. Maybe I'm onto something, maybe I'm just mad at the state of modern MTG and its bullshit. Either way, Veteran's Powerblade is a terrible card, and I can't think of a viable reason why it exists.