In Brief: Sailor Mercury's first attack may be known as the weakest out of all the attacks but it's also one of the most useful. While most of the other Scouts' attacks are offensive, Sailor Mercury's Mercury Bubbles Blast is one of the few that can be used in a defensive style by creating a mist to hide her and her team-mates from the enemy, give them a chance to escape and/or confuse and infuriate the enemy. Mercury Bubbles Blast has also been used to create a kind of bubble shield around Mercury to protect her from lava (!) in the first season finale and also boost her protection from a fire attack in the final season, Sailor Stars.
Even after gaining much more powerful attacks, Sailor Mercury would still, very often bring out Mercury Bubbles Blast due to it's practicality.

Cultural: "To see bubbles in your dreams, represent merriment, fun, and childhood joys. It may also symbolize wishes or unrealistic expectations. In deciphering this dream symbol, consider also the phrase of "bursting one's bubble".

To dream that you or someone is taking a bubble bath, represents ultimate relaxation. You have rid yourself of your worries/difficulties and release all the negative emotions you have been keeping inside."

~ BrilliantDreams.com

The Attack: In the anime, Sailor Mercury begins the attack by falling back and appearing to summon a powerful bubble into her hands. Major bubbles begin to spin around her and a bubble begins to form in her hands and starts to glow. As it glows, Mercury spins around, releases her hands from the bubble which is now floating on it's own, and breaks it with her hands and sends all the bubbles or mist towards her target.