Collecting unusual objects is more widespread than you might think, and there can be various psychological factors at play. For starters, we have an evolved tendency to accumulate resources, and whereas this used to be food or tools for survival or status, today it manifests in more idiosyncratic ways – such as erasers.
Your partner’s collection might be akin to an adult ‘comfort blanket’ – while all around is in constant change, the collector retains complete control over their growing collection. A collection can build up over years or even decades, and this seeming permanence can also provide consolation against existential anxieties. In fact, many collectors will make careful plans for what happens to their collection after they die.
The focus of your partner’s collection might strike you as strange, but erasers make an appealing item. They’re small, cheap, come in infinite forms, are multisensory (I’ll bet he has some scented erasers) and can be imbued with memories from the collector’s childhood, for instance.