pros:

• the most obvious benefit is of course like you said the health benefit this brings
• educational benefit since most people do not know how much water they should drink at least

cons:

• if you give it a thought it is quite strange to think we need to rely on our apps notification to drink water
• since many factors are involved in how much water you should drink I believe it won't be that accurate in some situations (like on a day you do a ton of sport or going out for a swim,...)