It's mainly because of my OCD, and so that I know I didn't lose a marker. However, you can use a #1 marker in a #2 slot to signify that you're upgrading a level 1 spell to level 2 to gain extra effects. This requires that you know you're going to use an upgraded spell slot ahead of time, however.
For example (taken from a D&D 5e forum post):
When you cast a spell using a slot of a higher level than the spell — for example, casting shatter (a second-level spell) using a fifth-level slot — the spell may have increased effects. Shatter does extra damage when cast using a slot higher than second level. Other spells, such as comprehend languages have the same effects regardless of the spell slot used to cast them
It's mainly because of my OCD, and so that I know I didn't lose a marker. However, you can use a #1 marker in a #2 slot to signify that you're upgrading a level 1 spell to level 2 to gain extra effects. This requires that you know you're going to use an upgraded spell slot ahead of time, however.
For example (taken from a D&D 5e forum post):
When you cast a spell using a slot of a higher level than the spell — for example, casting shatter (a second-level spell) using a fifth-level slot — the spell may have increased effects. Shatter does extra damage when cast using a slot higher than second level. Other spells, such as comprehend languages have the same effects regardless of the spell slot used to cast them