You will get better at the specific singing tasks or repertoire that you repeat a lot. If these tasks work with your voice to make it stronger, more flexible, more in-tune, more anything-desirable, then we could call that “productive practice”. How productive? That’s the big question.
You may practice diminuendos a lot and become able to make your voice go from loud to very soft on a single pitch. You have achieved success with a “volume task”. But what if the voice gets more breathy as it gets softer? What if the vowel becomes unintelligble with a dynamic change? How about if 90% of the diminuendo can only happen on the last 10% of the duration, rather than evenly through the duration?
You may practice range extension and get another two half-steps on top by pushing a little harder. The two new notes don’t sound like the rest of your range, but they are there. Now you have succeeded at a pitch task but at the expense of which other things? Dynamics? Vowel? Pain? Endurance?
A technical “breakthrough” can be something that makes all of your singing easier, or it might be something that makes one area surge ahead, while the rest of the voice trails behind. This is why a teacher with excellent listening skills is required for the best progress, for the majority of singers. A good teacher will help you to evaluate the costs associated with a specific strategy or technique. They can help you to develop in a balanced way.
As a singer becomes more highly trained, they can take over more of the listening and teaching for themselves and rely less on a teacher, but even for a very self-aware, well-trained singer, assistance from an excellent teacher or coach can save valuable time.
Having a set of technical domains in which to work is an important part of keeping balance when practicing. Agility, dynamics, legato, staccato, and vowel intention, are all important for the voice’s health. All of these domains may not be represented equally in the music you sing, but keeping your voice healthy is a slightly different matter from practicing your music. Excellent, specific technical work + your repertoire preparation with intention and desire to say something = singer’s progress.
If you ignore what the voice needs for fitness, you may have problems. “Just singing” may keep things working for a while, but not at your fullest potential forever.