system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] What is the tactical or mechanical advantage to piloting walkers if you also have access to speeders and tanks?

Saw lots of empire walkers the other day. Including some single-rider models barely larger than tauntauns. Some speeders whipped by them going like 10 times faster. And there were some tanks too, hovering over the uneven terrain just fine. 

So...why bother walking and running when they can fly?
gpt
You probably saw an AT-ST. they are garrison vehicles, primarily intended to control and supress crowds, not be at the front lines.

Antigrav is expensive and are more complicated to maintain compared to walkers. This is bad when you have got them on planets all over the galaxy, many who don't have people to maintain advanced tech.

Finally, Repulsorlifts doesn't make the vehicle weightless. A more heavily armored vehicle, would require a bigger, more expensive repulsorlift system. Engineers usually go for more shielding instead of just armor, but these, like repulsorlifts, require specialized crew to maintain. 

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Like you said, the advantage is speed, but that is irrelevant in many cases. Walkers are intended to move slowly alongside infantry, and support them. 

Finally, there are many planets where repulsorlifts simply don't work. 
Electric storms, electronically charged mud, hostile repulsorlift jammers, and other bullshit like that would severely cripple an army that is dependent on repulsorcraft, and leave the men without armor support..  
When you are deploying troops all over the galaxy, you gotta be prepared for all situations.

**TL;DR: Different vehicles fill different roles. There are no reasonably priced "perfect tank"**