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I use intervals.icu to chart fitness, TSS, freshness etc. With enough data it does a very accurate job of estimating TSS from heart rate in outdoor rides with no power meter. I’ve also been swimming this week while on holiday, want to have a vague idea of TSS from that to see how it affects my recovery week - so far so good on that front too. Grabs all its data from strava so I’ve been manually adding swims.
It doesn't really matter but when you look back in 3 months it might give you some insight into why event x went so shit or whatever knowledge you're trying to extract from historical data.
It's nice when comparing season to season to go "oh, maybe that's why that event was so shit compared to the year before" etc.