whats the significant benefit of miniLED?
Most significantly, better black levels and contrast, due to finer grained local control of the LED backlights.
Basic LCDs have a single backlight that lights up the entire screen, so blacks end up looking like dark greys. Computer IPS screens are generally built this way. A more advanced LCD will have multiple backlights arranged in a checkerboard pattern, that can be individually controlled. The Apple XDR monitor is built this way. With this technology, black areas can be black and bright areas can be bright because the brightness of the local backlights in those areas would be set differently, appropriate for the content on screen at the time. A mini-LED has even more backlights arranged in a finer checkerboard pattern, again with each backlight that can be individually controlled, and there may even be thousands of them in the screen size of a large iPad.
I'd be very tempted to wait for mini-LED, because right now the biggest drawback I find with the 2017 and 2018 iPad Pros (and probably the 2020s as well), is the mediocre black level performance. However, I'm not sure I want to wait that long, and we have no idea how great the mini-LED performance will be if/when it appears in the iPad Pro. Will it be a big upgrade, or will it be a marginal upgrade? The thing about mini-LED is it could be anywhere along that range. I suspect both power utilization and cost will factor into the decision by Apple on how far they want to go when exploiting this technology.
EDIT:
BTW, for reference, I'll provide some ballpark brightness and contrast ratio numbers:
iPad Air 2: 420 nits max brightness, 0.39 nits min brightness, 1000:1 contrast ratio.
iPad Pro 2017: 600 nits max brightness, 0.37 nits min brightness, 1600:1 contrast ratio.
iPad Pro 2018: 600 nits max brightness, 0.37 nits min brightness, 1600:1 contrast ratio.
Apple XDR monitor: 1600 nits max brightness, 0.04 nits min brightness, 40000:1 contrast ratio.
iPhone OLED: 600 nits max brightness, 0.0 nits min brightness, ∞ contrast ratio.