The most common, and fastest way, to begin building your real estate sales business is through your SOI (Sphere of Influence). It makes perfect sense, the people who already know you, like you, and trust you will usually be the ones who typically do business with you first. This is certainly a pool of business that you must take full of advantage of.
However, if you're looking to grow your practice you're going to need far more people than just your SOI. The people you already know tend to lock you in at a certain number of units and price points you can sell every year, and unless you're SOI is sending you an endless amount of referrals, you're going to need to leverage your resources in order to grow bigger.
That's where marketing and advertising becomes vital to your growth. Most real estate agents absolutely HATE to spend money! They don't like to discipline themselves to allocate a certain amount of money from their commissions in order to keep adding "wood to the fire" and as a result they depend solely on market influences such as waiting for someone in their SOI to reach out to them, phone duty, open houses, and prospecting efforts that require no money but plenty of man hours spent.
Unfortunately, there simply aren't enough hours in a day to meet or speak with the number of people you need to in order to grow a real estate business. Therefore you have to do what the rest of the agents are unwilling to do - spend money.
Spend money marketing and branding yourself. Spend money on advertising to the consumer where they are. Then watch every dollar you spend like a hawk. Keep doing what's working and cut whatever isn't.
The bottom line is, if you're going to actually build a real business you can't depend on your SOI to do it for you. Your SOI is a support but they're not meant to be the leverage.