A lot of people who play poker on the Internet like to focus on tournaments.
They either play lots of fast-paced sit'n'go tournaments, or they play in normally structured multitable events.
These "normal" tournaments usually have massive fields, and the player base of the sit'n'gos is very large as well, meaning it is often very hard to get to know the tendencies of your opponents – other than what you've seen on that specific day.
But because I have always tried to stay one step ahead of my opponents by making slight adjustments to my play every time it seemed they thought they had figured me out, because of this I have no doubt whatsoever that playing against the same small group of core players has been an important part of my winning poker strategy.
Now that I have made the transition to Internet play, it is not as easy here to get me in the same kinds of favorable positions.
Because online I cannot see my opponents, it is much more difficult to analyze any tells or habits that my opponents may have.
Also, these tells / habits are a lot less reliable than in real life – if only for the fact that sometimes two or three people play under the same account (making it even harder to get a good line on one's play).
And while in my normal limit hold'em or pot-limit Omaha game in my regular cardroom it would be an exception to be up against more than two unknown players, on the Internet it is entirely possible to be at a table full of unknowns.
This is especially true on some of the larger sites (PokerStars, PartyPoker), where the player base is so large that it is simply impossible to get to know every single player.
But the note-taking feature that most sites offer does make this process a lot easier.
Every time that you're in a game and you notice a very good or very bad play by someone, every time that you see patterns in one's play, you can simply write that down – something that is not allowed in brick and mortar setting.
Now, the next time that you will be playing at the same table with this person, these same notes will still be there, so you will know immediately: "Hey, this is that guy who calls raises with king-ten offsuit", or "Oh, that's the fella who only bets made hands and who almost never semi-bluffs".
This type of information is just invaluable, and even more so if you are someone who (like me) actively seeks to play in games with specific players, while avoiding others.
If you play on sites where the player base is not that large, or if you like to play certain types of games that have a relatively small core group of players, then not taking notes would be the same as leaving money on the table - something unforgivable for the professional player.
While most sites have a very large number of limit and no-limit hold'em tables, at the game that I like best (pot-limit Omaha high) the player base is usually not nearly that big – even at the larger sites.
This means that within a couple of months, one should be able to gather quite a bit of useful information to create a list of notes and records; a list that is extensive and reliable enough to give the creator of this list a significant edge in the game.
Of course, it is important to be very accurate when taking notes, and to analyze correctly the specific situation that may have led to the good or bad decisions you have noticed.
You should take into account the size of the game, the type of game and whether or not the play you witnessed is 'standard' for this specific player.
You should try to figure out the level of your opponent's knowledge, any betting patterns that may give away the strength of his hand, whether or not his play deteriorates whenever he's losing, etc. etc.
Basically, you do exactly the same as in a regular cardroom – the only difference is that online you write things down immediately, while in B&M cardrooms you would have to wait until you get home to make this analysis.
All in all, it should be clear that this note-taking feature is yet another advantage for those people who play poker in a systematical and analytical manner, and that the people who don't care about these things "because people who know their players don't need to write stuff down" will be at a distinct disadvantage.
In fact, they are losing out on a simple but excellent opportunity to keep both the quality of their play as well as their overall results at the highest possible level.
To sum this up taking notes while playing online is something ever poker player should do.