Very often there is a pair of HN's at HEL, might be due to e.g. increased risk level for breaches (for example when they know some neighboring countries are having larger exercises over Gulf of Finland etc.) or for preparation of some training session near the sea etc. and they bring the planes to HEL from TMP.
But as they regularly keep them at HEL, naturally pilots likely need also to practice landing to HEL on time to time so I'd say seeing HN at HEL is just normal occurrence, not necessary even requiring any specific reasons.