i just wander, if google has its playstore, why should it created app downloader, i just dont get it. i mean whenever i need an app or game, i go straight into store to choose and download it, but making a downloader make it so redundant. not just google i see others doing the same thing too.
So if I understood your question and confusion correctly, you are asking:
"Why is there a separate GOG Downloader, when you can already download/install/play games with the Galaxy client?"
First you must realize that the GOG Downloader existed long before the Galaxy client, and that you can connect to the GOG store and your account also with a web browser, not only the Galaxy client.
At first when GOG.com was new, the only way you could download your games from them was with your web browser. You can still do that too.
Later, they created a GOG Downloader tool which makes it somewhat easier to download the games from your account. You still had to use it with your web browser, ie. you click on a "GOG Downloader link" on your web browser, in order to initiate the download on your GOG Downloader client. A bit like how you click on a torrent link in your web browser and the download starts in your Bittorrent client.
Then, later GOG came up with the Galaxy client, which pretty much made the GOG Downloader client redundant and obsolete. GOG hasn't supported the old GOG Downloader client for a long time now.
So nowadays you have two (or three) official
ways to download your games from gog.com:
1. Downloading the standalone offline installers with your web browser, and install the game with them.
2. Using the Galaxy client, and click on "Install" button on it, which downloads and installs the game for you, Steam-style.
The third way is to download the standalone installers with your Galaxy client (using the backup option), and then installing the game manually with them. So basically the same as #1, but you are using Galaxy as your downloader client, instead of your web browser.
For the record, I wrote this message with my web browser (Chromium), not the Galaxy client.