The real advantage of keeping those old textbooks around is they give you something to fall back on when you hit the inevitable wall where an app just doesn't explain *why* something works the way it does. Grammar books actually break down the logic behind rules in a way that sticks with a lot of people - especially if you're trying to understand deeper patterns rather than just memorizing phrases for the next lesson.
That said, there's a pitfall people don't always catch: textbooks from even 10 - 15 years ago can feel outdated in terms of how language is actually used now. Slang changes, word frequency shifts, and the way native speakers communicate (especially online) moves pretty fast. So they're great for the structural foundation, but if you're only using them without any exposure to current media or conversation, you can end up with knowledge that feels stiff. The sweet spot is mixing them with apps, podcasts, or content in the actual language - use the textbook as your reference guide when something doesn't make sense, not as your only source.
If your books are reasonably recent and cover the language you're learning, they're definitely worth keeping. Just don't expect them to teach you everything solo anymore. The grammar explanations are the real goldmine; lean on those while getting your listening and conversational practice elsewhere.