Buying Alderweireld makes sense because, as we are well aware, Spurs need another centerback. Whether you believe in Michael Dawson as the partner for Jan Vertonghen, that Younes Kaboul will be fit enough to play like he did in 2011-12 or Etienne Capoue is adequate cover, Tottenham can use another player in central defense and Alderweireld is a well-regarded 24-year-old who is the of interest to a handful of Premier League clubs.
Eriksen is a more surprising target for Spurs, though. The club already has Lewis Holtby, Gylfi Sigurdsson and even Moussa Dembele as creative midfielders so why bring another one in?
Spurs' interest may simply come down to value. Eriksen is one of the more highly touted young players in football, with 36 caps for Denmark at just 21 years old and has been in the crosshair of Borussia Dortmund, Bayern Leverkusen and AS Monaco this summer. Spurs may not need Eriksen, but if they think they can get him at a good price (£15 million for the pair is great value, even if you aren't as high on them as most people are, like me), that might be enough for them in this spend-happy summer.
Of note, Liverpool reportedly want the Ajax pair as well and signing players they want is fun.