Not everyone is familiar with what Open Source is, or why it is important for web development.
In the good-ole-days, you would contact a web development company and they would build you a website. Their code would be written from the ground up, entirely bespoke and custom for your requirements. And that's fine, except that most of your budget would be spent on things like payment processing integration, listing pages, shopping carts and so on. And, if you wanted to part ways with your chosen web dev company, you'd have to find someone new who would be willing to take over their system - if you were allowed to continue using it at all.
With Open Source, hundreds (sometimes thousands) of developers work together on a project, making the code available for free to anyone who wants to use it. It is maintained, managed, evolved and improved by those same developers. And that means you spend no money re-inventing the wheel, building another shopping cart or handling payments through Paypal. Instead of spending money on the basics, you can concentrate on building a site that appeals to your audience. And if you part ways with one Magento development company, any other Magento company can take over in a flash.
Want to run four shops with the same product? Want to use different currencies on each one? Want to keep stock in sync across all of them? And want to do that without needing to build and maintain four different codebases? No problem, Magento does that out of the box.
Most platforms allow you to extend their functionality with modules, extensions or add-ons. Very few do it as well as Magento. Extensions work within their own "space" in the code of your site, meaning interference between extensions is rare. Extensions are easy to upgrade, enable, disable or hide, and most allow you to control who can access the extension within your admin area.
All e-commerce systems need some modifications out of the box. That's inevitable - search engines are changing their algorithms and ranking criteria nearly constantly, and software takes time to update. But Magento starts very well indeed - decent URLs (with canonicalisation baked in), meta tag control, site maps, great template structure in the default themes, and even mobile friendly in the more recent versions. And if that's not enough, there are plenty of free and paid extensions you can grab to tweak every aspect of your site's SEO efforts.
Most people don't intend for their e-commerce shop to stay the same size. The aim is always to grow the customer base, process more orders, pull in more traffic. But not every system grows micely or smoothly as your store increases in size. Magento is built to scale, and it does so brilliantly. You can start with a single server, and expand as you need to - another web server here, a load balancer there, and without any need to rewrite code or plan for hours or days of downtime.
Magento includes a bewildering array of features out of the box, from an extensive access control system to email template management, content page editing, multiple currencies, tax zones, customer grouping and group discounts, flexible special offer pricing and a huge collection of payment providers, shipping methods and an extensive API. And what it doesn't support out of the box can be added through the excellent Magento Connect extension marketplace.