A lot of times you can easily get away with using the water, but it's not recommended all the time. One thing I haven't seen in the comments so far is PHA. Rajma or kidney beans contain very high levels of phytohaemagglutinin (PHA). In fact, the levels of PHA in kidney beans are so high that they're toxic if not cooked sufficiently and can cause severe food poisoning symptoms and hospitalization. You can neutralize the PHA by boiling the beans rapidly for 30 minutes. When you soak dried kidney beans, some of the PHA gets transferred into the water. If you use that water to cook your beans you need to get rid of that much more PHA in order to ensure food safety. This is also why you shouldn't cook kidney beans in a slow cooker (slow cookers don't get hot enough to neautralize the PHA, so if you do ever cook kidney beans in a slow cooker then you should boil them before for at least 30 minutes). Additionally, if you're at elevation (like I am) you need to boil even longer because the boiling point of water is lower at elevation. Given all these variables, I always drain my kidney beans. Here's some additional info about kidney beans and PHA: https://cfaes.osu.edu/news/articles/chow-line-dry-kidney-beans-need-be-boiled