Independent of esthetics and chemistry, help the creature to get out of the bath.
On occasion i) a round bottom flask accidentally slides out of your hand and drops into the bath. Or ii) a clip fails to keep it attached, e.g. to the optional splash guard (for smaller flasks, it simultaneously can be a helpful "extension" to reach the bath) and either bath liquid enters the evaporating flask, or/and product spills into the bath and you want/must recover your product by extract from the bath. Based on personal experience, such a rescue is easier if the bath is kept clean of floating/dissolved/decomposing residues to then requiring "only" a couple of extractions of the cooled bath (and only little additional purification [wash/back extraction/drying, recrystallization; distillation]) than an additional chromatography on top.
If there are many samples in small round bottom flasks to concentrate (say of a volume $\pu{25 mL}$, or less), you may mount them to the evaporator with the splash guard and an additional adapter (for the narrow neck). You may fill the bath to the upper mark which usually is fine/acceptable until you mount a flask with larger displacement without an adapter ($250, 500, \pu{1000 mL}$). By immersion of these, your bath now can overflow. The rotation of the flask equally lifts and sprays some of water, too. Now you/your colleagues have to mop up the bench/floor again and again instead of doing something more useful (update the log, mount/dismount a column, etc) in parallel to the ongoing concentration. It is not so much about the performance of the bath running during the day typically around $\pu{50 ^\circ{}C}$ (if it is water* based); because if you need to rotavap large quantities, there are larger evaporators (e.g., R-220 ex with a $\pu{20 L}$ evaporating flask), too.
The stains the picture depicts equally could be by partially corroded (metal) Keck clips introduced over time e.g. if you mount a small 25 mL flask to the evaporator via splash guard and an additional adapter for a more narrow neck.
* Tap water often contains too many minerals (hard water) to be a useful bath liquid in the long run (think removing the deposits with vinegar like for an electric kettle). Some groups hence use softened water. Quoting the English manual of the R-100 by Buchi (the first company successful to commercialize rotary evaporators) advises e.g.
5.8 Setting up and filling the heating bath
Do not use pure distilled water or de-ionised water.
If pure distilled or de-ionised water has to be used, add 1 to 2 g of Borax ($\ce{Na2B4O7 x 10 H2O}$) per litre of water.
[...]
The recommended fluid for use in the heating bath is water. Normal tap water may be mixed with distilled water with a mixing ratio of up to 1:1 according to the water hardness.
(p. 25 of the manual, revision 09.2022)
