Firstly, moral support... He's such a legend in Croatia that just having him around is a boost.
Secondly, they might think that, in clutch moments, a retired Goran Ivanisevic is more likely to come through than, say, Roko Karanucic or Lovro Zovko, who are the 4th-ranked Croatians in singles and doubles, respectively. (Karanucic standing at 164 in the singles entry rankings and 429 in doubles, Zovko being 114 in doubles and 449 in singles).