This is something I've noticed among a lot of Samsung users; "don't use the samsung messaging app, get Textra", etc.
I've been using both extensively and yet I can't see what's wrong with the stock app. 0 issues with either. However, with Textra I have to pay a couple of bucks to get rid of ads so I'm back to the stock app. Screw that kind of practice.
Convince me, why should I choose one app over the other?
Well, for me it's all about customization. And Textra just works and the developer is always extremely responsive and friendly if you ever encounter any issues.
Here is another reason I like Textra - persistent reminders and customization per contact.
For example, if my wife texts me, it will remind me 10 times every two minutes in case I stepped away from my phone and didn't check it when I came back.
Yes, I know I can set notification reminder in Accessibility for the default app, but if I use that it only uses the default phone notification (not the notification tone set within the app) and it is a blanket reminder, meaning it will do it for every single message not checked. I don't need it for every message.
My default is to ring and vibrate for all messages, with one reminder at 2 minutes if I haven't checked it.
For a group chat with my siblings, who love to text at all hours, I have that conversation muted where I will get a different colored notification icon at the top of my screen, no LED flashing light and no ring or vibrate. That way I can still see if I get a message, but it won't bug me unless I look at my phone.
In Group Messages, I can set the contact color for each person, so that the message bubbles are a different color for each person.
As others have said, being able to reply on the lockscreen w/o ever having to unlock the device is one of the main reasons. I hate having to unlock the device just to respond with Yes or something simple.
Now, as for the cost let me give you a tip. Download the app Google Opinion Rewards. it gives you occasional surveys and as you complete them (typically no more than 1 or 2 questions), it gives you a credit in your Play Store account to use. Just be sure you are honest in your response. For example, if it asks if you have been to a few places and you haven't, don't say you have. If you do, you will stop getting surveys. I started using this app approximately 4 years ago and I have now accrued over $150 in Play Store credits. This is great to use for app purchases, in-app purchases, movie rentals, movie purchases, etc.