1: Only to faciltate announcing to install it and monitor for updates. Like any other app, you can do it manually, but with less convenience.
2: What added functionality does it bring to your end-use requirements? Yes it is very powerful and extensible, but often equipment vendors pride themselves in offering a complete solution to their customers, enhancing their product range and functionality continually, not only hardware but software. This u$ually cones at a cost, sometimes upfront, but increasingly on an ongoing basis as well. Did I mention HomeAssistant is free? Often the quality of the alternative offered by HomeAssistant is higher, allows for older equipment to continue to work, allows for interoperability with different vendor solutions tied together for powerful integration and automation. Cashflow and profits are vital for vendors, whereas Open Source solutions can provide a viable free option.
3: You have up-front costs for your HomeAssistant server (a small computer), whether it be a Nabu-Casa green, a Raspberry Pi, a NUC or other hardware, or optionally a virtual environment, running a software device that pretends to be a computer on your existing equipment. You need to provide for internet access for updates. You need to have a robust network for your cameras and other devices to connect to the server, whether wired or wireless. You access your HomeAssistant server via a web browser or Companion app you run on your Android/Apple platform. Electricity costs to run everything are small, but ever present and ongoing 24/7/365. Your time is free if you are futzing around, tuning and updating constantly. If you need to pay somebody to do that for you, the support charges mount up. The learning curve is steep, rewarding, and oh-so-tempting - you’ll wonder where all the hours have gone! The commercial arm of HomeAssistant exists to provide funding for services, hosting, some hardware, and remote access solution. Optional but an easy way to show your support for one of the busiest and popular Open Source projects around
4: A pencil sharpener for your fingertips may be an option if you are contemplating extensive customisation. Most modern devices and software are designed to easily integrate, plug-and-pray and rosary beads optional, and should be recognised automatically. Matter is a recent industry standard innovation, requiring a one-off onboarding operation, most elegantly performed through a mobile device like your android phone with QR code scanning making it easy. Setting up monitoring, updates, customisation, automation involves a bit of mousing and typing, best remotely done from a desktop with keyboard and mouse and a web browser.
The HomeAssistant server itself runs on a flavor of Linux. It can be hosted on a number of different environments, even virtual ones, such is the flexibility of Linux. It is designed to run headless, where you access it over the network, locally, or (with optional configuration and possible cost), remotely over the wider Internet.
The power of HomeAssistant is Open Source, where people give you free support in the forums such as here, constantly expand and develop the ecosystem to integrate solutions from different vendors, without vendor lock-in and cloud dependence, so you can monitor and control everything from one place - making you an accomplished puppeteer, pulling all the strings at one time, confidentally.
I recommend you read the HomeAssistant introduction on their website, examine if the hardware you are contemplating purchase is already supported to fulfil your needs, and ask your wallet what it thinks.
Best wishes.