Why I need a landing page?
Let's start with the question: what is a landing page?

A landing page is a stand-alone web page that is designed specifically for a marketing or advertising campaign. Its purpose is to persuade visitors to take a particular action, such as making a purchase, filling out a form, or signing up for a newsletter.
The design of a landing page typically focuses on a single “call to action”, which is prominently displayed on the page and reinforced through persuasive copy and imagery.
Actually it is any webpage focused on one offer.
I’d say: Do you have one button with “call to action” on page? You have a landing page.
Why I need it?
Landing pages are often used in digital marketing campaigns, such as pay-per-click advertising, email marketing, and social media advertising.
You need a landing page on your website to increase your conversion rates and maximize the effectiveness of your digital marketing efforts. By providing a clear and focused message, you can guide visitors toward taking a specific action that aligns with your business goals. Landing pages also allow you to track and analyze the success of your marketing campaigns, so you can make data-driven decisions to optimize your results.
What to keep in mind
When creating a landing page, there are several key factors that you should keep in mind to ensure its effectiveness. Here are some important considerations:
Purpose:
Clearly define the purpose of your landing page and the specific action you want visitors to take. This should be reflected in your headline, subheadline, and call to action.
Audience:
Know your target audience and create content that speaks directly to their needs, interests, and pain points. This includes using language and imagery that resonates with your target audience.
Design:
Keep your design simple, visually appealing, and easy to navigate. Use attention-grabbing headlines, subheadings, and bullet points to make your content easy to scan and understand.
Call to action:
Make sure your call to action is prominently displayed and stands out from the rest of the page. Use action-oriented language and create a sense of urgency to encourage visitors to take immediate action.
Trust:
Build trust with your visitors by including social proof, such as customer reviews, testimonials, or certifications. This can help to establish credibility and encourage visitors to take action.
By keeping these key factors in mind, you can create a landing page that effectively communicates your message, inspires action, and generates results for your business.

But you can fail in case you ignored the Key Factor
Tracking, analysis and improvement
Continuously test and refine your landing page to optimize its effectiveness. This can include testing different headlines, colors of call to action buttons, and even page layouts to see what resonates best with your audience.
How to do it properly? Google Analytics 4 is a powerful tool for such purposes. I hope you have already migrated from Universal Analytics.