The only reason investors (and especially VCs) are listening to, or reading your pitch deck is because they want you to take their money and make more out of it. Therefore they are looking for substantial market opportunities.
With your market slide, you give specific numbers about how big your company can get (often referred to as TAM - Total Available Market) and whether there are any specialties in the market. We encourage you to put some decent research into these numbers and back them with legit sources (... and add them to the presentation). There is nothing worse than presenting wrong market information and having an investor listening, who knows that. On the other hand, a good market slide shows the investor that you are the right person for the mission because you can make good decisions based on your extensive knowledge.
Investors are particularly interested in substantial market opportunities. One example is a market that is going to change because of external factors like regulations or new technology. A recent example of this is the opening of the German long-distance bus network to private companies that allowed "Flixbus" to become a unicorn. Another thing investors love is growing markets, so if you have a growing demand in your market, add the growth figures.
If you are founding in a new market, it can be challenging to build your market slide, because there is no data available. In this case, it is always a good idea to look at other industry sectors for analogies and to illustrate with numbers from the "old-market" you are competing with. An example that's often used is from Uber. They decided to compare themself to the "taxi-market" although they brought an entirely new service to the people and now there is a ride-sharing market.
A word about TAM, SAM, and co…
Some of you guys will probably have a big question in front of their heads right now. What's that TAM you are talking about. This is why I will give you a short explanation of the most important things.
TAM - Total addressable market
The TAM gives an answer to the question: "How many people are there and how much are they willing to spend (e.g., the education market was around $1.3B in 2017).
SAM - Serviceable addressable market
SAM represents a smaller part of this market that we are hoping to serve with our product (e.g., the K-12 education market).
In most cases, you should add a projection to your pitch deck market slide with something like. We are hoping to get a 10% market share of our SAM in 2025.
