BETTER VARIETY OF GAMES - Online there are usually games being played of any type at any level at any time - from Omaha to Hold'em, from $.01/.02 up to $20/40.
NO-LIMIT CASH GAMES - B&M casinos typically don't set up no-limit cash games because in no-limit games luck becomes less of a factor and skill tends to dominate. Therefore, these games tend to break up pretty quickly because the fish bust out too fast and the games dry up.
MORE TOURNAMENTS - B&M card rooms only have tournaments at specified times on specified days - usually only a couple per week. But online there are many more multi-table tournaments and ongoing (unscheduled) sit-n-gos.
BONUSES - Because of the lower cost, online poker rooms give away tons of money in bonuses to sign up and make an initial deposit at their site. Most bonuses tend to be about 10% of the initial deposit. Some sites also offer reload bonuses, which are bonuses offered to players who already have accounts open. In most cases you have to play a certain number of hands before cashing out the bonus and/or wait 30 days before cashing it out. Always read the terms and conditions of the bonus offers.
LOWER RAKE - Most B&M card rooms charge a 10% rake but most online card rooms charge a 5% rake. The lower cost of playing definitely adds up in the long run - especially at low-limit games where the rake, as a percent of the amount of the pot, is larger.
NO TOKING - You don't have to tip the dealer. This adds up in the long-run.
MORE HANDS PER HOUR - Most B&M card rooms will deal about 30 hands per hour but online card rooms will deal about 60 hands per hour (even up to 100 hands per hour in shorthanded games).
BAD PLAYERS ONLINE - Bad players are sometimes annoying when they play brick & mortar because their bad play and suckouts get criticized by good players. The anonymity of playing online allows them more comfort and allows you to make more money.
PLAY MULTIPLE TABLES AT ONCE - Many low-limit players like to play many tables at once - sometimes up to 4 or 8. Not only does this increase your win rate but it also smooths out the profits because you are playing around 200-400 hands per hour.
EASY TO SPOT JUICY GAMES - Almost all sites tell you the number of players seeing the flop, the avg. pot, and who the players are at the table. This information can be used to find the most profitable tables.
SMOKE-FREE - You don't have to worry about people smoking around you.
WOMEN WON'T GET HARASSED - Some women don't like playing poker with guys because they are sometimes rude or disrespectful to women. When you play online you can pick a screenname which keeps your sex anonymous.
BEGINNERS WON'T FEEL INTIMIDATED - Beginning players sometimes feel embarrassed when they play live and they make mistakes or think too long about what to do. Online people won't be able to tell.
NO DEALER MISTAKES - Dealer mistakes can distort your results in brick & mortar games. These mistakes don't happen online.
LESS TRAVEL TIME - You don't have to spend an hour driving to and from the casino. This gives you more time to play the game. This also allows you to have short sessions when you sometimes feel like playing for only 20 minutes or so but wouldn't drive to a casino just to play for 20 minutes.
LOWER WAITING TIME FOR TABLES - Most of the time there are no waits for a online room (occasionally you may have to wait 5 minutes) but when you play brick and mortar games you could be waiting up to a couple of hours.
ABILITY TO KEEP BETTER PROFIT RECORDS - Because you have close access to a computer, you will be able to more accurately record the details of your sessions - the amount of time you played, the starting and ending chip amounts, and game limits.
HAND HISTORIES - You can a review the play of a hand by having a hand history e-mailed to you.
OBJECTIVE ANALYSIS - You can software that imports hand histories and analyzes and serves you statistics about your play.
CALCULATING POT ODDS - Almost all sites tell you exactly how much money is in the pot at any given time. This allows you to precisely calculate the pot odds and implied odds at all times. You do not have to keep track of how many bets went into the pot like you do at a B&M card room. This will result in more correct odds-related decisions.