[Samsung Galaxy Tab A series ROMs, Kernels, Recover] K9100ii:
I would strongly recommend doing so, and it would be a necessity if you've got a particularly old version of the stock firmware.

If you're still on an older version of the stock firmware with no modifications, to update, you can, of course, simply go ahead and use the built-in software updater for the first time to upgrade to the latest version of it without doing anything additionally. That would be the easiest way to go about it.

Alternatively, assuming you would do so using a computer running Windows, here are some basic instructions on how you would go about doing so:

You need two tools:

1. A tool called Frija. Use it to download a zip archive containing the latest firmware for your country. You need to specify a CSC for the firmware ("Consumer Software Customization"/"Country Specific Code") (You can find which CSC you need using an app like Devcheck (System -> Original CSC in it), or maybe it's somewhere in the "About tablet" section of settings in the stock firmware. There are also lists of CSC's you can search for that you can get the CSC for your country from.), which can be downloaded from here: https://xdaforums.com/t/tool-frija-samsung-firmware-downloader-checker.3910594/.

2. Another tool called Odin to install it. I've got my copy of it from here: https://build.nethunter.com/samsung-tools/Odin_3.13.1.zip

After downloading those tools and the stock firmware using Frija, you should be able to go about installing it by basically doing the following:

Extract the downloaded zip archive containing the stock firmware.

Open Odin, and select the extracted .tar.md5 files for the corresponding slots - The one with the filename beginning with "AP_" to the AP slot, "BL_" to the BL slot, and "CSC_" to the CSC slot. Ignore the file with the filename beginning with "HOME_CSC_" - Using the one with the filename beginning with "CSC_" will get you a clean install with your data formatted, and using the one with the filename beginning with "HOME_CSC_" will not get your data formatted.

Connect your tablet to your computer while it's booted into download mode (Although I'm assuming you know how to, just for information, power it off, and hold Power + Volume Down + Home, and press the Volume Up button to confirm the warning. Just for even more information which might end up being useful, to get out of download mode for whatever reason after accepting the warning, you can hold down Power + Volume down for several seconds.)

Wait for Odin to detect your tablet (it should be nearly instant unless there are drivers to automatically install), and start the installation, making sure not to accidentally disconnect your device.

Note that when it reboots, if you were running the stock firmware and were signed into a Google account, you'd need to connect to a WiFi network and enter the credentials for it, unless you've disabled Factory Reset Protection by enabling the "OEM unlock" option in developer options.

If Odin doesn't detect your tablet while booted into download mode, you may need to install the drivers from here: https://developer.samsung.com/mobile/android-usb-driver.html

Let me know if you struggle with it if you go with this route.