Diversity is key when it comes to money, especially as you get more of it. Having a single checking and savings account is fine. But in regards to your investments, you should consider a mix of index funds, mutual funds, stocks, bonds and real estate.
Not every investment is going to get great returns all of the time and since you don't control the economy, you have no way of knowing which one you should put all of your proverbial eggs in. By having your money spread out, you are pretty sure that you'll never lose all of it in one fell swoop. Your international market index fund might fall, but the North American one might be perfectly fine. There might be a major round of layoffs in your community, causing people to sell their homes, but that means you have a larger applicant pool for your rentals.
You don't have to jump in and diversify to this extent at once, just increase your diversity as you increase your wealth.