Lots of growth stocks are experiencing a volatile year in 2021, following an incredibly good one in 2020. When stocks are gyrating this much, it can be difficult to stomach being an investor. It may also offer an opportune time to buy stocks you've had your eye on. This is especially true for growth stock investing because the price of these stocks generally moves more than the overall market.

Airbnb (ABNB 1.03%) and Roblox (RBLX -1.07%)are two such growth stocks I have had my eye on, but haven't bought. They each have excellent long-run potential and a business model that people find intriguing. Still, I am near the max level I allow as a rule to allocate to stocks in my portfolio during a bull market, and so I'm not buying these stocks just yet. 

However, if the stock market crashes by at least 15%, I will consider adding these two growth stocks to my portfolio. 

A declining stock chart.

Image source: Getty Images.

1. Roblox 

Roblox is a human co-experience gaming platform with 42.1 million daily active users, up from 23.6 million last year. It's especially popular with kids and over 50% of its users are under age 13. Users on the platform can customize their avatars and play games together with friends. The platform is free to join. However, some items and games will be restricted to free players. The only way to access these areas and items is to pay with Robux, an in-game currency that must be purchased with real currency, and that's how Roblox generates revenue. This is also referred to as a freemium business model.

That business model will change if management's plans come to fruition. In the future, Roblox revenue will come from transactions (as it is currently) and advertising. The supplemental revenue stream would boost the company and allow it to monetize players who never deposit any money on the platform. 

Despite being a growth stock, Roblox is making transformative progress in generating free cash flow. In all of 2019, Roblox had a free cash flow of $14.5 million. In 2020, that figure increased to $411.2 million. Admittedly, 2020 was an abnormal year driven by surging engagement as many of its users were urged to stay home because of the pandemic, but that's incredible growth nonetheless.

That cash can be used to further Roblox's efforts to expand by attracting an older demographic (as it is making good progress on already) and adding potentially lucrative advertising capabilities to its platform. 

This company looks to be on the right track, and I wouldn't mind an opportunity to add Roblox stock if a broader market crash comes along. 

Two kids laying on the floor playing on a tablet.

Image source: Getty Images.

2. Airbnb  

Airbnb competes in a large hotel and resort market estimated to total $1.22 trillion in 2021. That market was devastated by the effects of the coronavirus pandemic and is only recently starting to recover. Still, restrictions on international travel are common even as COVID-19 vaccines get distributed more broadly. As more people get vaccinated (nearly 2 billion vaccine doses have been administered) and travel restrictions ease, the pent-up demand for travel could be unleashed.

One of the benefits users of Airbnb cites over hotels is convenience. This is especially true if you are traveling with family and friends and would like a larger space. On Airbnb, you can find an entire home available for rent for the week or maybe even for the month. It would be difficult finding something equivalent to an entire home offered by a hotel operator. 

Moreover, on Airbnb, you can find rooms and homes for rent in areas where a hotel room is not even an option. These advantages should allow Airbnb to take an increasing share of travel spending over time. Airbnb's gross booking value in 2020 was $23.9 billion -- a tiny sliver of the overall lodging and resort market.

In addition to offering a product that people love, Airbnb's business model allows it to offer more of its product without large capital investments. If Airbnb notices a surge in demand and wants to increase supply, it doesn't need to spend hundreds of millions building structures. Instead, it targets investments in bringing on more hosts and incentivizing them to list more rooms and more often.  

Airbnb offers a product that's better than its competitors. Its business model allows it to expand and contract along with consumer demand. And it stands to benefit as the world recovers from COVID-19. This is one growth stock I would love to own when the market gives me an opportunity.