The boot.img itself will not give you root, it's just so encryption isn't forced-used either to decrypt the phone, or keep it decrypted when flashing new builds (the stock ones force encryption on first boot). Even after flashing new custom kernels, you will still need to flash supersu or your preferred root method afterwards. Every time you flash a new boot.img or custom kernel, you will lose root and have to re-flash supersu all over again.
I only mentioned root in the OP because I wanted folks to know that in 7.0 and higher, so far only systemless root is working with these (I could be wrong, but I think that for the most part people seem to prefer systemless anyway). I hope this helps clarify.
Maybe I need to edit the OP if it's misleading.
Sorry I will try to be as clear and precise as I can. I did following steps.
1. Flash stock images
2. fastboot format userdata
3. fastboot flash boot boot.img and fastboot flash recovery twrp.img
5. Reboot into recovery
6. Flash some mods e.g. Surround sound, Pixel Launcher Zip, Ad Blocking Hosts, etc.
7. Dont flash root.
9. To my surprise, no modes works. I see ads, no pixel launcher , no surround sound etc. However it is still decrypted.
Now repeat these steps but also flash supersu. All mods working now.
So my problem is that mods dont stick without root.
However on tigerstown modified boot.img and some other on that thread, , I didnt need to root for my mods to stick.