It is because the "Doklam" is not far from Indian territory, and in particular is not far from the so called "chicken's neck".
In the image below (see the bottom of this answer for a more informative map):
- The so-called "Doklam plateau", territory officially recognized as disputed by Bhutan and China, is circled in black (see also the other map further down the answer)
- the "Doklam" mentioned in the question, and main point of dispute, is the area that extends from the above territory till the triple junction between India, China, and Bhutan just south of it
- India's "Chicken's neck" circled in red
For India this "neck" is a strategically important piece of land, that (in case of open conflict with China) if it would fall in enemy hands would cut land connection to India's north-east, meaning that India would probably lose that territory in a war.
So by keeping China away from the border, India keeps them away also from its weak spot.
Moreover, as mentioned by Olivier Pucher in the comments:
[the] game of provocations and squizzing some military advantages is much more subtle that direct attacks. Clearly China doesn't intend to invade India, neither from its own soil nor from Bangladesh (which probably wouldn't be more practical at all), but it tries to put pressure on India. If Doklam's plateau is recognized as chinese, chinese army will be able to manoeuver there at will.
Given all the confusion about the location of "Doklam" I'd like to highlight this passage from wikipedia:
In June 2017 a military standoff occurred between China and India as China attempted to extend a road on the Doklam plateau southwards near the Doka La pass. Bhutan has formally objected to China's road construction in the disputed area.
I.e.: China is building a road through the plateau area. A road that would allow China to easily reach the triple junction, and thus have a military advantage in case of confrontation with India.
Thanks to Olivier Pucher for this other image coming from this other website: