Here are some examples.

Restaurant
A restaurant in a small town estimates that there are 60,000 people in the neighborhood. They have 5,000 unique customers in a year. They estimate market penetration as follows: P = (5,000 / 60,000) x 100 = 8.3%. The restaurant decides that level of penetration is too low and decide to invest in local advertising to reach more customers.

Business Services
A SaaS company has identified 5,000 corporate customers as its target market. They currently have signed 50 customers. They calculate market penetration as: P = (50 / 5,000) x 100 = 1.0%. The company decides to expand distribution to reach a broader target market.

Fast Moving Consumer Goods
A research firm estimates the percentage of total customers in a nation that have purchased a variety of branded consumer products each quarter. Companies use this information to evaluate customer base and develop strategies in areas such as advertising and promotion.