OK, so I hear this instruction used by teachers over and over again. And consequently by voice students as well without any idea of what "raising the soft palette" actually accomplishes. It is really not complicated, but even so most I have spoken to do not seem to know what it actually accomplishes in reality.
First, raising the soft palette simply lowers the larynx. And lowering the larynx causes the singer to elongate the vocal tract. Funnily enough the teachers that criticize singers for having "too dark" of a sound are the ones telling the same singers to "raise the palette". Ironically that will make the sound darker. Comical and sad at the same time. Comical in its irony and sad because the student is being pulled in different directions. On one hand they are told their sound is too dark and on the other hand they are then instructed to do something that makes it darker even though the teacher wants it to be "brighter".
Secondly, when a singer does lower the larynx that can be a good thing, IF AND ONLY IF it is what that particular singer needs at the moment. If a singer comes in with a depressed larynx whereby it is already too low, then instructing them to raise the palette will only make it worse.
Third, one of the hardest things to get as a singer is chiroscuro. That is the balance of "clear/bright" to "dark". The clarity/brightness comes from the core/squillante in the sound. That is produced by the thyroarytenoid muscles (chest voice) and approximation of the vocal folds - as well as proper resonance tuning. The scuro or "darkness" comes from lowering the larynx. What often happens when the larynx is lowered, whether by the instruction to lift the palette or "yawn", the student usually ends up distorting the vowel and thus the "clarity/squillante" or chiaro. So the singer has to learn how to *properly* lower the larynx without depressing it and or losing the clarity of the vowel or "squillante".
Lastly, keeping the concentration on the palette being high can eventually cause laryngeal depression whereby the larynx is too low. It usually also causes the hyoid bone to be pressed onto the thyroid cartilage. So there is a loss of flexibility in the voice to do fioratura, dynamics and also clarity.
The irony never ceases to amaze!!!