It's seen as a good suggestion for the class. For example, a Bard is given weapon choices, and rapier/longsword are probably better choices than any simple weapon:
(a) a rapier, (b) a longsword, or (c) any simple weapon
Barbarian gets a choice of a greataxe (which is martial) or any other martial melee weapon, but greataxes are commonly used by Barbarians.
(a) a greataxe or (b) any martial melee weapon
For the Sorcerer specifically, the proficiencies make the choice a bit more interesting:
PHB page 100:
Proficiencies: Weapons: Daggers, darts, slings, quarterstaffs, light crossbows
The choice of "a light crossbow and 20 bolts" or "any simple weapon" could be seen as a suggestion to take the crossbow, or another simple weapon if desired. Most Sorcerers are only proficient in Daggers, darts, slings, quarterstaffs. However, if you were to choose an Elf with proficiency in shortbows, you could start with this since you have proficiency. A Dwarf proficient in Handaxes could make this choice and be proficient in it.
Additionally, a Sorcerer could choose one of the other simple weapons and just not get a proficiency bonus in it. Perhaps the sorceror is Human and picks a feat such as Weapon Master (PHB: 170) and picks up proficiency in a simple weapon that a Sorcerer does not already have proficiency with. But if this Human Sorcerer picking Weapon Master also chooses to be proficient in Heavy Crossbows, could not choose to start with one of these due to it not being within the Sorcerer starting kit (feel free to work with your DM or come up with a background reason, or spend some of your starting gold on it).