Amazon's annual Prime Day, actually 48 hours this year, certainly boosts memberships — and by extension the value of the company — but the e-commerce giant also needs to focus on serving brands, according to analysts.

"If you look at the Prime membership, each year Amazon makes it more and more valuable ... which is ultimately what's critical to creating shareholder value," Neuberger Berman's Daniel Flax told CNBC's "Squawk Box" on Tuesday.

In the first 24 hours of Prime Day — which began at 3 a.m. ET on Monday and ends at 2:59 a.m. ET on Wednesday — Amazon said worldwide sellers saw their biggest one-day sales in company history.

During last year's 36-hour Prime Day sale, Amazon said Prime members bought more than 100 million products.

Michael Graham of Canaccord Genuity also emphasized how integral Prime membership is for Amazon. "At Amazon, growth is slowing," he said on CNBC. "It's important to make sure your customer lifetime value is as high as it can be."

While paying Prime members differentiate Amazon from its competitors, Graham said the company still has work to do with its third-party brands. "Amazon is not brand-friendly," he contended. "They've underperformed in things like retail and home, where brand is a little more important. They haven't been able to really crack the code there."

"That's an important strategic issue for Amazon going forward," Graham stressed, considering that as powerful as Amazon is it faces stiff competition from traditional brand-name retailers and smaller upstarts alike.

In fact, the Prime Day event also boosts online sales of its rivals. According to Adobe Analytics, retailers that have more than $1 billion in annual revenue saw a 64% increase in their digital sales compared with an average Monday.

Keybanc's Edward Yruma also believes that Amazon is putting all its emphasis on the Prime membership. "They're trying to reduce churn. You're creating excitement around the event."

Asked whether he would buy shares of Amazon, Walmart or Target, Yruma said on CNBC that he sees "incredible value" in Target. "They've made some incredible inroads with their products. The stock is the least expensive among the three of them. And they've finally fixed this online business."

Amazon did not immediately respond to a request for comment.

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